But why? Nothing happens if a Report is marked as Bad. I used to think I should try to avoid them (for esthetic reasons), until I realized it's actually good to test the limits of what gets deleted once in a while.

It's trivially easy to find old worthless posts: just go to the start of the Altcoin boards and click one. But reporting them doesn't matter at all, nobody reads there, so it's a waste of everyone's time to go through them.

I do report older posts once in a while though, for instance when an old spammer became active again. Or if it's spam: deleting those post removes one more backlink from the internet.