Why is there need for synthetic fiber?

Answer 1

Whether there is a 'need' or not, synthetic fibres often possess certain qualities that natural fibres do not.

There may also be economic/social reasons why synthetic fibres are produced. Nylon, for example, was invented as a substitute for silk. It is far cheaper to produce and has superior qualities such as in the manufacture of parachutes. The days of silk parachutes have long gone, sad but true. But think of all those happy silk-worms!