The King of Jerusalem granted the Holy Orders territory within his realm to bulwark his position. Holy Order forces were regularly called upon (and were a major component of) the kingdom's army during times of crisis.
And herein lies the bonus - it is significantly cheaper to hire&maintain the Holy Order "mercenaries" if they're granted territory in your realm.
At least, that's how it's supposed to work (and a quick check with the 3rd crusade bookmark suggests it works in that case) - if you have any savegames that show contrary behaviour they'd be greatly appreciated.