I can only get lump and imagine that briquettes would be more uniform in shape and therefore burn more evenly, and ignite the briquette next to them more uniformly. Because of the random shape of lump, I find you can have too few points of contact between a burning ember and the next piece, which means it doesnt catch as fast, and it is this that gives the varying temperatures I think. Also, lump is not all the same - one part might be more dry than another next to it.
I have often wondered about making my own briquettes, but never gotten around to it yet.