Why do circuses have clowns?

The lure of the circus is its spectacular displays of human skills and daring which amaze and thrill the crowds, when the excitement becomes too great for the audience, it is eased by the clowns. By making the spectators laugh, the clowns help them to relax again.

Oleg Popov

Oleg Popov of the Moscow Circus is one of the greatest clowns ever to have worked in the circus. The role of he plays is that of the simple bumpkin trying to copy the great riders and acrobats. His efforts to imitate their acts almost seem to succeed. But in the end they are always bungled!

Reference: This and That, Who and Where.