Thomas and Ludanto have answered the initial question as to why followers are valuable.
Reynard, you are correct that only henchmen can be vassals/underbosses. There is nothing preventing you from having one of your preferred followers be a henchman, though.
“It seems a bit strange that a character’s vassals should be limited so sharply by his Charisma, or that a character at his henchman maximum would be, for example, unable to accept the surrender and pledge of fealty of a minor princeling.”
The limit on the number of vassals being equal to the number of henchmen is a game mechanic designed to emulate the decentralized power structures that existed in most times and places historically.
Within the game world, political situations might arise that do not precisely mesh with the game mechanics. The best way to handle those situations is in the short term to do what seems logical within the game world and in the long term to assume that the game world will eventually adapt itself to the realities of the game mechanics.
For example, let’s assume that Marcus has his maximum number of vassals (6). He now accepts the surrender of a subjugated baron, giving him seven vassals. Rather than say “you can’t do that because that means you have too many vassals,” the Judge allows Marcus to have a seventh vassal.
He then secretly makes reaction rolls for each of the six vassals. He notes down which vassal had the worst reaction; this vassal is outraged that Marcus did not assign the new baron as his sub-vassal, and as a result he becomes a traitor. In the next month, he only pays 75% of his taxes. The month following its 50% and he starts talking to adjacent lords about a possible change of fealty.
The maximum number of vassals rule is thus, really, the maximum number of vassals a PC can sustain while knowing that “the rules have his back”, e.g. as long as he doesn’t demand more duties than normal from his vassals, he can count on a measure of loyalty. But more than that and he’s opening himself up to treachery, infighting, and drama.
(Think of it as similar to the situation where a Charm Person spell has worn off. If he’s halfway intelligent, the formerly charmed victim is not likely to all-of-sudden reveal to his master that he’s no longer charmed. It would require role-play to figure this out.)