Hi
My question is why we provide neutral pole in some circuit breakers?what's the purpose of neutral?

Be more specific, which circuit breaker do you have that has a neutral? Normally the neutral is not switched or broken. There are very specific cases that the neutral would be switched. Or are you referring to a Ground Fault Breaker, which has a connection for a neutral?

Really need more detail with your questions.

Internally on GFCI Breakers need to sense the difference of current drawn between the hot and the neutral.