It was to prevent Hojo's private power from intervening in the shogunal administration and also to show that the sovereignty of the shogunate resided in Seii taishogun (literally, "great general who subdues the barbarians") and Gokenin who was in a master-servant relationship with the Shogun.
北条氏の私的権力の幕政への介入を抑制するために幕府の主権が征夷大将軍とその主従関係下にある御家人にある事を示すためだった。