Do I really need an overgrip?

I'm using a size 5 prince O3 original red racket with it original grip for about 3 or 4 months now. My tennis instructor mentioned an overgrip once or twice after I got this racket. The racket fits perfectly in my hand and the grip never slips due sweat during play. So do I really need an overgrip? I don't want a racket grips that's too big. Also if I should use one, what should I get?

i never used an overgrip either. i dont see the point. im happy with the replacement grips.