If you got a "spot" on the tonite show, you were seen by a sh*tload of people and, well, you were almost set.
I only know of Carson through reading "Confessions of a late night talk show host" the "autobiography" of fictional talk show host Larry Sanders. Johnny, according to the book, gave Larry his break and inpired him to get into the talk show business. iirc, this is partly based on the reality that Garry Shandling had his break on The Tonight Show (possibly also guest host, I'm not sure.) There is an obvious affection for Johnny in the book and, even though it is a story, I have a feeling it is heart felt by the writers.
There are probably other comedians and actors I admire who hit the big time after being introduced on the Tonight Show, but that's the one I know about.
-zudo