D&C 107:18 The power and authority of the higher, or Melchizedek Priesthood, is to hold the keys of all the spiritual blessings of the church—
19 To have the privilege of receiving the mysteries of the kingdom of heaven, to have the heavens opened unto them, to commune with the general assembly and church of the Firstborn, and to enjoy the communion and presence of God the Father, and Jesus the mediator of the new covenant.
Why would we care that the Melchizedek priesthood holds the right to commune with the members of the church of the firstborn?
Incidentally, it is clear from that verse that the church of the firstborn, as referred to here, does NOT just mean the members of the church. It is ridiculous to say otherwise. Do we think one needs the melchizedek priesthood to have the right to commune with the regular church members?
But back to my main point. One might as well ask why a nonmember would want to commune with missionaries from the church. He would want to commune with members of the church so that they could bring him into the true church, teaching him what was required and then making him a member if he will conform his life to it.
Likewise, we would be interested in communing with the church of the firstborn so that we could learn what is expected of them, and so that they could teach us and we could come up to their standard of knowledge and living, and eventually invite us to become a member ourselves.
In short, how could we ever become part of the church of the firstborn unless we were taught what was expected of them? And who could teach us that except those who belong to it?
Thus the ministering of angels is no academic matter. It is not mortal men that we must commune with. The priesthood is completely unnecessary in order to bring mortal men and women to commune with other mortals. But to commune with the general assembly, and the church of the firstborn, that might require the power of God.