Are there any incentives to building new holdings?

[Tegus]

I'm sorry if this is a stupid question. I can't seem to see the purpose of creating new holdings under most circumstances. If you build a holding only to grant it to another character, then the return on taxes would be quite miniscule. I could only see myself building a castle for myself if I was unable to expand.

Building holdings is mostly useful if you build them and keep them yourself. For instance, in one game I played as the Duke of Saxony and had three castles on Sjaelland. This in turn meant that I could put down my marshal/steward to train troops/collect taxes, and their passive bonus(more levy/more tax) actually affects all three holdings instead of a single castle.