D&C 64

It is after the Prophet Joseph Smith returned to Kirtland, that the Lord gave reasons why He commands certain things.  The first in regards to displinary actions the Lord had previously given in His Law -

"...if he or she do any manner of iniquity, he or she shall be delivered up unto the law, even that of God...
89 And if he or she confess not thou shalt deliver him or her up unto the church, not to the members, but to the elders. And it shall be done in a meeting, and that not before the world.
90 And if thy brother or sister offend many, he or she shall be chastened before many.
91 And if any one offend openly, he or she shall be rebuked openly, that he or she may be ashamed. And if he or she confess not, he or she shall be delivered up unto the law of God."  (Doctrine and Covenants | Section 42:87 - 91)

Obviously, it was given to the priesthood leaders in the Church to administer disciplinary action as guided by the Spirit.  Again the Lord taught -

"...him that repenteth not of his sins, and confesseth them not, ye shall bring before the church, and do with him as the scripture saith unto you, either by commandment or by revelation."  (Doctrine and Covenants | Section 64:12)

In other words, in discipline, we follow the direction of the prophets and the Spirit of the Lord.  The Lord then explains why -

"...this ye shall do that God may be glorified—not because ye forgive not, having not compassion, but that ye may be justified in the eyes of the law, that ye may not offend him who is your lawgiver—
14 Verily I say, for this cause ye shall do these things."  (Doctrine and Covenants | Section 64:13 - 14)

As we consider discipline, it is critical that as we desire to show love and compassion, we do not offend the Lord as we do.  It is His will we are commanded to satisfy.

The second "why" comes with a commandment to Sidney Gilbert fater returning to his business and hiswork in Zion -

"And that which he hath seen and heard may be made known unto my disciples, that they perish not. And for this cause have I spoken these things."  (Doctrine and Covenants | Section 64:19)

I believe that the Lord would have all men teach and testify that which they have "seen and heard" to the disciples of Christ in order to build faith that the disciples perish not.