Makes it harder to what compunds are indigenous and what you just brought yourself. No use shouting Eureka over some organic compound in a hole you just spilled oil in.

In general great effort is made to ensure that probes don't contain any biological contaminates so if we do discover signs of life we're very certain it wasn't a microbe that hitched a ride.

As far as contamination from drilling (due to lubricants or something else entirely) I imagine that the desire for general cleanliness is for much the same reason. If we're going to spend hundreds of millions of billions of dollars on a probe to another world we want to be really confident anything interesting we find is actually from that world and not Earth.