The clay slows the ball down to a point that your opponent has a better than average chance of being able to get to the ball to make a reply. In addition, red clay courts make the ball bounce higher than hardcourts. The Western forehand grip was made for the high bouncing ball. Any kind of spin is exaggerated on a red clay court. Topspin bounces higher, kick serves jump out and out ferociously, slice stays low and at times it just dies.
Flat shots are relatively useless on a red clay court.
Green clay courts are similar to red clay courts. The big serve can still be useful on green clay, but it is not quite the weapon it is on hardcourts. It is extremely difficult to change directions quickly on clay court. Usually, the quick change results in slipping, and a resulting delay. Many clay court players slide into their shots.