Rollers allow runners to self-treat injuries, useful for myofascial trigger points (sensitive, tight spots in muscles). They should be used with care: on the right injury at the right time, in its healing phase.
It takes effort to hold your body weight while you roll your legs on the roller (quads, hamstrings and calves especially) but it helps relieve pain and sore tight spots, and can prevent tension building up. Alternatively, you could also try rolling out your aching muscles with a 2-litre bottle of frozen water.
Upper and lower leg muscle strains or pain, and you can also roll your back out on it too.
Quick, simple and portable option and a great choice if you want to control your own intensity level - you can roll as hard or light as you wish.