Wild guess, but if she's a true succubus, then manga logic dictates she feeds on lewd energy. Therefore, turning boys with succubus blood into girls and setting them loose sets up a network of lewd that leads back to her. It's like an MLM scam, but with gender bending. This would establish her as the demon matriarch of her own clan of lesser succubi.
Alternatively, she could just think that it's funny.