2 Nephi 31:4-8

4  Wherefore, I would that ye should remember that I have spoken unto you concerning that prophet which the Lord showed unto me, that should baptize the Lamb of God, which should take away the sins of the world.
5  And now, if the Lamb of God, he being holy, should have need to be baptized by water, to fulfil all righteousness, O then, how much more need have we, being unholy, to be baptized, yea, even by water!

2 Nephi 31:4-5 (Emphasis mine)

Nephi(1) makes a particularly important observation that we should remember.  Christ had to be baptized to "fulfill all righteousness."  I find this an interesting phrase.  What exactly does it mean?  Elder Cecil. O. Samuelson gave a talk about this phrase at BYU in 1995.

"Assuming that we would also like 'to fulfil all righteousness.' we need to clearly and more fully understand what this entails. 

"Righteousness has, I suppose, come to have several meanings, but that to which I refer and believe the prophet was describing is that which is 'acting in accord with divine . . . law' (Merriam-Webster's Dictionary). 

"Clearly, because the Savior was sinless and without blemish, fulfilling all righteousness requires much more than just the avoidance of sin. The righteous life then cannot be a merely passive existence but obviously requires active performance and behavior, which has never been promised to be easy ...

"Most of us will not, nor are we expected to, fully appreciate the implications of all of our decisions or actions. Likewise, not all of our choices or deeds will have the same level of impact or influence on ourselves or others. The challenge, however, is that we cannot know, usually, in a prospective way what it is that will make a real difference in our personal lives or in our efforts to serve and bless others. As a result, if we are prudent, we must really act and understand that all we do has the potential to make the real difference for us and for others. Likewise, we must accept the potential reality that what we do not do will also have possible geometric repercussions for many more, and in many more ways, than we might estimate. 

"As important as our good intentions are, in the last analysis it is what we do or do not do that determines real outcomes in our quest to become more like Jesus. 

"These sobering reflections alone might provide a basis for doing all that we can to 'fulfil all righteousness,' because even though the Lord knows the end from the beginning, we are most often not privileged to know now. Admittedly, we do need to do whatever is necessary to avoid the "big" mistakes, but we cannot afford such preoccupation with only escaping sin and bad decisions that we are deflected from doing the good we must do and making the hard but necessary choices we must make ...

"Many of our responsibilities, opportunities, challenges, and crises are uniquely and individually crafted for our own growth, development, and testing. For those that are without direct parallels which might be observed in the lives of others around us, special and individual responses are necessary if we are to be successful in meeting them. Happily, whatever the specific challenges, we have been promised sufficient help to overcome or meet them if we do our parts faithfully ..

"The commandments or 'rules' of God, if you will, have not been given to capriciously or arbitrarily or thoughtlessly vex or punish us. Rather, they have been provided to us as protectors, guides, leaders, teachers, and loyal friends to ensure we have what we need to 'fulfil all righteousness' ...

May we live so that we can do all the things--and especially those that seem only 'little' things--that are so necessary for us  'to fulfil all righteousness.' " (Emphasis mine)

Keeping the commandments, avoiding sin,  and being ready for anything we could face in life are ways to "fulfil all righteousness."  I liked his comment about not knowing which of the things we do will have a profound affect.  We have to treat all challenges as well as opportunities in life the same.  We must treat them as though they will make a difference.

Christ was baptized because it is required of all men.  And Nephi(1) points out that, if Christ, being holy, had need of baptism, how much more need do we have.  Christ's life was an example of how we should live ours.  This is the point Nephi(1) is making.

13 ¶ Then cometh Jesus from Galilee to Jordan unto John, to be baptized of him.
14  But John forbad him, saying, I have need to be baptized of thee, and comest thou to me?
15  And Jesus answering said unto him, Suffer it to be so now: for thus it becometh us to fulfil all righteousness.  Then he suffered him.

Matthew 3:13-15 (Emphasis mine)

5  Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God.

John 3:5 (Emphasis mine)

6  And now, I would ask of you, my beloved brethren, wherein the Lamb of God did fulfil all righteousness in being baptized by water?
7  Know ye not that he was holy?  But notwithstanding he being holy, he showeth unto the children of men that, according to the flesh he humbleth himself before the Father, and witnesseth unto the Father that he would be obedient unto him in keeping his commandments.
8  Wherefore, after he was baptized with water the Holy Ghost descended upon him in the form of a dove.

2 Nephi 31:6-8 (Emphasis mine)

Nephi

30  I can of mine own self do nothing: as I hear, I judge: and my judgment is just; because I seek not mine own will, but the will of the Father which hath sent me.

John 5:30 (Emphasis mine)

21  ¶ Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven.

Matthew 7:21 (Emphasis mine)

17  If any man will do his will, he shall know of the doctrine, whether it be of God, or whether I speak of myself.

John 7:17 (Emphasis mine)

29  And he that sent me is with me: the Father hath not left me alone; for I do always those things that please him.

John 8:29 - 30 (Emphasis mine) 

Nephi(1) gives us a powerful truth.  Christ was baptized to fulfil all righteousness.  He was an obedient Son, keeping his Father's commandments.  Christ did that constantly throughout His life.  He is our example; His path will lead to eternal life.   All this is done because it was His responsibility to give us the example of how to live.