Memory management takes the form of optimism, they'll not clear down memory in case the user asks for the same thing again. For people with lots of RAM (8GB+) this is fine.

However if you have a 4GB computer and generally single task, then a memory clean tool can work in your favour.

I have several virtual machines that start up overnight and close down in the morning, at that point I want my memory flushed to start a fresh with my applications not the VM's cached data, which wont be needed again for another 12 hours.

This is of course a corner case and just last week I upgraded to 16GB of RAM mitigating the requirement. But there is no one size fits all memory manager out there.