there is need for co-ordinated turns in RC with certain aircraft. (cubs for example) the reason that you need to do this is called "adverse yaw". when you use ailerons the plane wants to point the nose towards the opposite wing. if you watch closely yo will see the nose climb a bit when banking for a turn but once you release the bank it drops down again. using rudder eliminates that.

again some planes need more rudder than others.

using opposite rudder will put the plane into a "slip" this is were you cancel out the turnning caused by the bank and fly "sideways" this is a good way to bleed off speed during a deadstick or glide but should be practiced high up as it can lead to a stall if you arent ready for it.

its a good idea to practice using the rudder when you fly, just do everything a few "mistakes" high so you can be ready for the results.