Serious answer: because an entirely distinct biosphere would be a veritably goldmine for biochemistry research! We could discover enzymes that work under extreme conditions like high pressure, low temperature, all while bathed in a soup of chemicals toxic to terrestrial life. For all we know, we could discover a simple way to catalyse ammonia production at low temperatures and feed the world with cheap fertiliser! Or… who knows!? We won’t know until we go looking!