Free memory is wasted memory. You really don't want to have free memory lying around unused, until you really absolutely know that you are going to use it in the future and there is no way to evict existing things from memory in the meantime. In that case, you pre-allocate the memory to keep it available to you.

But yeah, if you intentionally keep free memory around, you are doing things wrong. It's lying around doing nothing, while you could put it to work on more useful things.