The tighter the balance of power is in the overall House of Representatives, the harder it will be for their most politically ambitious members to resign their seats and take jobs in a potential Biden administration (taking their similarly politically ambitious senior staffers along into the depths of the swamp with them). There’s value in this as well.
Not only that, the smaller their majority, the more prima donnas and others in the Democrat party can hold Pelosi’s hostage....and the harder it is for her to push through radical legislation.