Usually it's to wedge

not me tho, no couch, no way... I've always used a purpose built mount

I've heard (and definitely never tried this as a horny teenager left alone for the weekend) that you put a ziplock full of lotion/lube of choice between the cushions and use it as an improvised masturbation aid. I've also heard that it's more work than it's worth and much better results can be found using a couple of sponges, a few rubber bands, and the aforementioned lube filled ziploc