why do you think that some microorganisms produce antibiotics?

Answer 1

It is an adaptive process. The ones which adapt survive while those which do not disappear. It's part of the natural selection process. Adapt or die. That does not mean they do this consciously. 'Tis more a matter of happenstance. If you have trillions of microorganisms, one may come up with a defense mechanism, and that strain will survive and multiply.

Answer 2

to kill competitors, parasites or any other lifeforms that might pose a threat.