Mosiah 2:4-7

5 And it came to pass that when they came up to the temple, they pitched their tents round about, every man according to his family, consisting of his wife, and his sons, and his daughters, and their sons, and their daughters, from the eldest down to the youngest, every family being separate one from another.

6 And they pitched their tents round about the temple, every man having his tent with the door thereof towards the temple, that thereby they might remain in their tents and hear the words which king Benjamin should speak unto them;

7  For the multitude being so great that king Benjamin could not teach them all within the walls of the temple, therefore he caused a tower to be erected, that thereby his people might hear the words which he should speak unto them.

8 And it came to pass that he began to speak to his people from the tower; and they could not all hear his words because of the greatness of the multitude; therefore he caused that the words which he spake should be written and sent forth among those that were not under the sound of his voice, that they might also receive his words.

Mosiah 2:5-8

King Benjamin’s proclamation to his people has gone forth.  The entire family came and pitched their tenets. This was much like the children of Israel did in the Sinai.  “Each tent contained one family. Israel also pitched tents by families in the wilderness (Numbers 2:34). They dwelt in tents at the time they covenanted with God at Sinai (Deuteronomy 5:27-31).”^[1]

John Welch writes:

[A]t the occasion of King Benjamin's address, whole families were present. Benjamin caused all the people in his land to "gather themselves together" (Mosiah 1:18) and to assemble together (see Mosiah 2:9, 27), both Nephites and Mulekites (Mosiah 1:10): "every man according to his family, consisting of his wife, and his sons, and his daughters, and their sons, and their daughters, from the eldest down to the youngest, every family being separate one from another" (Mosiah 2:5). They assembled specifically "round about" the main Nephite sanctuary in the days of Benjamin, the temple in Zarahemla (Mosiah 2:6).[2]

When they pitched their tents, the opening was facing towards the temple to better hear King Benjamin.

8 And it came to pass, when Moses went out unto the tabernacle, that all the people rose up, and stood every man at his tent door, and looked after Moses, until he was gone into the tabernacle.

9 And it came to pass, as Moses entered into the tabernacle, the cloudy pillar descended, and stood at the door of the tabernacle, and the Lord talked with Moses.

10 And all the people saw the cloudy pillar stand at the tabernacle door: and all the people rose up and worshipped, every man in his tent door.

Exodus 33:8-10

John Welch further explains the purpose of Benjamin’s speech.

Ritual or ceremonial elements are especially prevalent in Benjamin's speech. The actual Sitz im Leben, or real-life context, of King Benjamin's speech can unambiguously be identified as ceremonial, since the people gathered around their temple to hear the speech (see Mosiah 2:5–6). If more were known today about ancient Israelite rites, more could be said about the ritual dimensions of the prophetic lawsuit in general, as well as the rhetorical significance of these factors in Benjamin's speech in particular. Under the circumstances, however, it is possible to assume that Benjamin's speech drew further rhetorical power from its use of ceremonial terms that were part of the covenant renewal process that occurred under the law of Moses on the Israelite Day of Atonement.[3] 

Traditionally, the people would meet within the walls of the temple.  There were so many people that they could not all fit within the temple walls.  Benjamin had a tower built so he could speak to the people.

The temple was used to impart teachings to the people.

It would not have surprised the Nephites that the Lord would choose to teach them at the temple. From what we know about their temples in the cities of Nephi and Zarahemla, these sacred places were obviously important religious and political centers for teaching (see Jacob 1:17; 2:2); people were routinely taught within its walls, see Mosiah 2:7); for preaching (see Alma 16:13); for imparting the mysteries (see Mosiah 2:9; Alma 12:9; 13:3, 16); for gathering for ceremonies, coronations, obligatory annual festivals, ordinances, and covenant renewals (see 2 Nephi 6—10; Jacob 2—3; Mosiah 1—6); for making royal proclamations (see Mosiah 2:30; 7:17); and for sacrificing "according to the law of Moses" (Mosiah 2:3).[4]

The construction of towers was not unusual. 

When King Benjamin crowned his son Mosiah, all the people in Zarahemla sat in tents around the temple while Benjamin addressed them. Because he "could not teach them all within the walls of the temple, therefore he caused a tower to be erected, that thereby his people might hear the words which he should speak unto them" (Mosiah 2:7). Was the need to improve the acoustics the only reason for the construction of this tower? Recent research has discovered ancient precedents for the use of such "towers" in royal convocations and coronation ceremonies. These biblical and Jewish precedents are not obvious to the casual reader and may well shed light on Benjamin's tower.[5]

There was much preparation for the sermon to the people.

For such a great day, sacred preparations were in order, especially those made by the high priest. Rabbinic writings report special efforts taken to keep the high priest awake during the night of the Day of Atonement, and pious men followed this example.¹⁰³ Benjamin's preparations, also, were substantial. He was awakened at night—"Awake; and I awoke. . . . Awake, and hear"—Mosiah 3:2–3)—by the visitation of an angel from God. He met with his sons (see Mosiah 1:10–18) and carefully wrote his speech in advance (see Mosiah 2:7).[6]

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[1] King Benjamin and the Feast of Tabernacles, John A. Tvedtnes, Maxwell Institute, July 2, 2014.

[2] King Benjamin's Speech in the Context of Ancient Israelite Festivals, John W. Welch, Maxwell Institute, accessed July 2, 2014.

[3] Benjamin's Speech as a Prophetic Lawsuit, John W. Welch, Maxwell Institute, accessed July 2, 2014.

[4] The Temple Context and Unity of the Sermon at the Temple, Maxwell Institute, accessed July 2, 2014.

[5] Insights 15/4 (1995), Maxwell Institute, accessed July 2, 2014.

[6] The Temple in the Book of Mormon: The Temples at the Cities of Nephi, Zarahemla, and Bountiful, John W. Welch, Maxwell Institute, accessed July 2, 2014.