Two simple reasons: 1. If there is life elsewhere, we want to make sure terrestrial lifeforms don't drive it to extinction before we get to see it. 2. We wan

Two simple reasons: 1. If there is life elsewhere, we want to make sure terrestrial lifeforms don't drive it to extinction before we get to see it. 2. We want to make sure that if life is found elsewhere that it isn't due to our own contamination of the site.