Why is it necessary to emasculate a bisexual flower. Mention the conditions where it is not necessary

emasculation is necessary  because it prevent self pollination. we are doing cross breeding for getting a good veriety. But in the presence of self pollination we cant obtain such a new veriety. that's why we are emasculating a bisexual flower.
if we are considering a female flower there is no need of self pollinatoin.