It's too ensure that all parties involved in space exploration are following the stipulations of the various treaties put in place. These treaties are there for a reason, one of which I've already mentioned is the cross contamination of planetary biology.
Lets say the ESA is spending billions of dollars on research on their moon.
A private company launches their rover to the moon, but didn't properly decontaminate, bringing along earth microbial life.
The ESA rover detects this life and now the ESA thinks they've discovered life on the moon. When in reality it was just traces left over from the US companies rover.