In Warhammer Online, Keeps are one of the main components in RvR battles.  Just one of the many benefits of controlling a Keep is additional over-time experience your guild gains.  If a Keep is under siege, you'll also see a convenient warning letting you know.

In WAR, guilds play an integral part in claiming keeps for their side. Not only can you fight together with your banner held high, but those banners can be used to benefit everyone in the area in keep defense once you’ve claimed the keep as your own. Not only that, but your guild will gain certain benefits and advantages just for owning a keep. Imagine your colors flying high for the masses to see. Stir passion in your allies and instill fear into the hearts of your enemies as they come to recognize your colors!

WAR Herald has a lot more about Keeps in this Guild Highlights article!