the taleworlds official forums have the 1257 ad faq on there that answers your question

the simple and quick answer is this
production of of goods that you can sell at major towns every week
local and always present ransom broker
a constable that can muster large support armies (from the manors population) for use to either help you take fortifications or set them to patroll your fiefs to prevent them being raided.

you will invest a HUGE amount of cash into this place but if your a effective slaver your ransombroker will return that cost very quickly.

for deeper information follow JCs suggestion

The main thing I use the manor for:

When the population reaches around 300, you can levy a huge army from the manor. The army will have a few top tier troops and loads of low tier. It follows you around or can patrol your territories. If you let the population reach 300 again, you can create a second one. now you have a 600 peasant death squad you can use to conquer castles with without lords' help.

If you build a monastary, you can summon a handful of top tier crusader knights into your army every so often.

oh my that's sounds orgasmically amazing, thank you much for the info fellow..mounters