Garrison Hall got unlocked when I lvl up my castle to 15, what is its use for farmers?

Garrison hall is useful for all players when you are attacking or being attacked. It will be of little use when attacking monsters but for any other conflict it can help. If you do not do any attacking (even in Reign of Chaos events).

Upgrading this building though (and the war room buildings that you will also be able to add) will give very good power points, very useful for building events in Clash of Provinces