I don't think you quite understand, so let me explain again, slowly.

We are able to pick up to 10 abilities, but only 5 are usable per synth. If you equip more than five abilities, five of them are chosen at RANDOM for you to use. If you equip only five abilities, you are guaranteed to get those five abilities all the time.

Why equip more than five abilities and randomize the skills you get? What is the point of the extra five skill slots?