Alright, from I have read in the manga. Yoko's main reason for collecting the stones is to establish a failsafe. She wants to eat humans more freely and without the backlash of other yokai from grouping together and toppling her altogether. By collecting all the fragments, her power grows from beyond controling human minds to control other other yokai.

Typical villainous fare, but its what Yoko did to Kon that was just fucked up altogether. In which Yoko ate every member of Kon's clan and brainwashed Kon into being her underling. Which made the abandonment that happened in the beginning somewhat more appalling.