I’ll be brief:
Carson is personally respected by the broadest of constituencies. His supporters at the ballot box are RELATIVELY few, but they are ultimately LOYAL to him and his ideals.
This makes his endorsement extremely important. It is even more important because it comes on the heels of what is clearly an attempt by Trump to pivot from bombastic to “presidential” and under control. Carson’s support will give this pivot validity to a lot of people.
And, yes, there will be a position in the Trump administration for Carson. But, I also think Carson is concerned about the destructive forces afoot in the Party by the action of the Party elites to undercut the will of the people.
I call them as I see them. IMHO - bottom line - Trump just solidified his stranglehold on the nomination. He will be the nominee. There will be no contested convention.
The idiot elites in the Republican Party - Romney, Jeb Bush, et al - by stepping in attempting to put their thumb on the scales of the nominating process, have done for Trump what he probably could not have done on his own. That is, begin bringing the Party together, but this time with the elites on the outside looking in.