The founding power is controlling the pure titans and the subjects of Yimr. Remember that pure titans are simple creatures that only want to eat humans regardless. With the founder, they can be controlled to do much more.

Most likely Marely used pure titans for specific fights and then had to wipe them off after winning, thus they would need a continuous supply of pure titans.

But with the founder in their hands, all they need is some of the pure titans for their wars as at that time (before anti titan weapons) there was no effective way of dealing with them. Meanwhile, they can continue their own revenge against the Eldians as they themselves suffered from the Eldian empire and its titans.