Overcurrent devices protect against indirect contact by EEBADS (Earthed Equipotential Bonding and Automatic Disconnection of Supply), basically if there is a fault to metalwork on a piece of equipment, current flows down the earth wire, the bonding reduces the potential between it and nearby metalwork and the MCB switches off due to teh large overcurrent.
RCDS are added in a few situations:
1) For suppimenary protetcion against direct contact for sockets that might be used outside, to give you a bit of proptection incase you have a momentry lapse in concentration and pick up a mowed over mower cable, etc
When the next version of the regs comes out, it'll be that all ogeneral purpose sockets have to be protected, not just ones that can be reasonably expected to be used for outside stuff
2) often required on an electric shower, consult the maker of the shower, and basically a very good idea where you have water, electricity and people together (and doubly so if they are wet and naked!)
3) If you have a TT supply (earth from a local electrode in the ground) then the earth path is of too high an impedance for this to work and RCDs are used to sense the imbalence
4) For added protection against fire in locations such as farms, or where the nature of stored or processed materials presents an increased fire risk (paper mills and the like)