The reason you want to hold a stretch for at least 30 sec is to give your fascia ( to quote Wiki here Fascia: “a sheet of connective tissue beneath the skin that attaches, stabilizes, encloses, and separates muscles and other internal organs) time to stretch, and to give  your muscles and joints time to increase their range of movement. Think of holding a plastic bag, the longer you hold tension on the plastic the more stretched the plastic becomes, this is what we want our fascia to do. If you go too far/deep in your stretch you actually trigger a myotatic stretch response and it will become counter productive). Using foam rollers can be great to further increasing fascia separation and elongation or they can be used to isolate muscle activation (if you roll out your glute prior to squats you will increase the activation of your glutes because you have increased the neuromuscular connection to this specific muscle, recruiting more fiber activation during the movement)