Leaving your memory with no use at all would be the bug. It is a free resource that could be used to improve system performance at basically no cost (the disk i

Leaving your memory with no use at all would be the bug. It is a free resource that could be used to improve system performance at basically no cost (the disk is already read, the memory is still available immediately for any app that request it, etc), so it is a good default to let linux do it. But could be cases where it could be side effects of it (i.e. running in a VM could make less memory available for the host, and it could be caching disk already) so you could consider to disable it sometimes.