What's the point of alliances with vassals/liege?

[Goodbye Horses]

Since you can't call them to help you in war (I don't understand why)

[Dave Mongoose]

A vassal in an alliance with their liege cannot join factions against them, but in exchange their liege cannot revoke their titles unless they commit a crime.

[Omar Comin']

If you are Feudal and you have any Tribal/Clan vassals, you also suffer an Opinion penalty from them if you are not allied.

[Ashantai]

It also means you can leverage all their levies in a war instead of just the smaller portion you get given.

It removes them from factions, yes, but it also adds to your strength rating to deter other factions.

[The Former]

Yeah, you can't call them up into foreign wars, but you can call them to help you crush rebellions of any kind.

[Goodbye Horses]

Got it, thanks