Board Thread:Suggestions forum/@comment-58.104.204.65-20180925135750/@comment-25981649-20190209115900

Personally, i assume it might give you small benefits towards that faction, such as crafting better items, and abilty to hire elite / special units (black-uruks, berserkers, gondor tower guards, southron champions, swan knights, rivendell elves, etc.)

Some factions would not allow you to hire any units without pledging. (Dunedain, High-elves, Half-Trolls, and when added, Moria)