people have explained it, but my way -
Example: Rally car drivers know that it takes to long to get around a courner the touring car way (brake, turn, accelerate) so instead, they drift. its all about control, and the more control your have the more skill you have in overal rc racing. its about entering a turn at at full speed and take the exit point earlier like in a drag- the slower the reaction, the bigger the time. to do this you have to slide so much that your car is paralel to the exit straight. normal rc car drivers may use the out-in-out tecnique while drifters will use anything more inwards like an in-in-in sorta thing or middle-middle-in. there are many different ways.
Most common reasons for Drifting:
1)Its fun
2)its about tecnique and style
3)it looks good and has better advantages
4)its faster that normal racing