General deterance. It makes it clear to others that destruction of evidence can be far worse than the crime you are attempting to hide.

"If only we lock up this one more person, _then_ it'll finally be clear."

How's prison working out as a deterrent, generally?

It works fine. Just because some people can't stop themselves from committing crimes, that doesn't mean the general public doesn't respect deterrents.

There are so many things I wouldn't do because I know I'd get in trouble.