Cary answered...

If baptism is the gateway to Heaven, then the baptizer is a border agent. Would you want any old person who claims to know how it's done to just start letting whoever wants to into your country? Why do you insist they carry a badge and wear a uniform, and have an ID which proves they are legitimate representatives of the government? It's the same way with God's rituals. All over the scriptures, whenever there's a ritual performed it was performed by a duly authorized representative. Doesn't it just make sense that if something essential for our salvation would be required, that it should need to be done God's way, by God's authorized representatives? Acts 19:1-6 even shows a time when the authority of the baptizer was in doubt, and Paul re-did the baptisms for good measure. With all the confusion that comes from competing people claiming to speak for God, or claiming to know His will in this matter or that, I'm glad Mormons teach about authority. There's so much more clarity about who we should listen to, and who might be perfectly well-intentioned, but who really shouldn't be listened to because they're not authorized.