Alpinists mostly.

Double rope for
Winding routes to reduce rope drag,
Belaying two followers at the same time on easy routes like Mt Whitney east
face, for speed and reduction of rock fall hazards or for training a new climber on easy terrain when paired with a experienced second.

Twin ropes for
ice climbers-lighter weight when you need full length rappels, less chance of cutting both ropes with axe or crampons.
Sport climbers working a route as twin ropes hold more falls before failure.

In the old james bond movie with the full length climbing fall scenes, I was told stunt man Rick Silvester used a pair of old ropes and tied them together with rubber bands to make it look like one rope so he wouldn't ruin a new rope. Effectively using a twin rope technique with old single ropes.

http://blog.owareusa.com/2014/01/24/castle-peak-on-donner-pass-california/