Why!!!!!!
Why not!! They are great fun, whats the point of S&L flight? If the wingtips, nose or tail are not pointing straight at the ground frankly you are wasting money.
Seriously though it teaches co-ordination skills and is also used for the collision avoidance maneuver. (and its great fun and the first steps towards areobatics).
Practise practise practise and you will be able to do them without looking at the instruments.