If you're seeing it on the rent/trade board, it's because users are 'required' to keep their threads active at least once every 7 days. If not, software automatically closes it as an abandoned thread.
This is on the rent/trade board only and not on any other boards. So on the rent/trade board you will see a lot of threads where the OP has 'bumped' the thread in order to comply with that board's posting requirements.