Programs often make time vs memory tradeoffs. In some cases, it is even possible for them to adjust these tradeoffs during runtime.
The most common example is is file-system caches. Pretending (for the sake of argument) that the kernel did not automatically cache the file-system, a program may reasonably make this optimization [1]. In this case, the program can easily release some memory by clearing its cache.
You could also be running a program with garbage collection that normally wouldn't bother doing a full sweep until it hit some memory usage threshold; again, it can do this on request.
I'm sure that people can come up with other examples.
[1] In fact, a program can request that the kernel not cache its file-system requests, in which case it would have actual reason to cache what it thinks it might need again.
Of course. But in practice here's what's going to happen. The good apps, say Postgres, will implement the ability to do this. They will create caches, and release them upon request from the OS. This will significantly hinder the performance of Postgres because it will keep losing its caches. Note that this is worse than just emptying the cache, you actually lose the allocation and have to start over. In some cases you'll effectively disable the cache, which is there for a reason!
Now, here comes, say, MongoDB, which says "Yeah, I have caches, and I need them all! I won't release any allocations because I have to have them." Let's say, for whatever reason you run both Postgres and MongoDB on the same box and it's running out of RAM. Now you are punishing Postgres, the good citizen, and rewarding MongoDB, the bad citizen, and only because Postgres bothered to implement the ability to give up its caches.
I cannot find the reference to this, but I believe this was tried in early Android, and the consequences were that it became unusable as soon as you installed at least one memory hog app that never gave up any memory.
The most common example is is file-system caches. Pretending (for the sake of argument) that the kernel did not automatically cache the file-system, a program may reasonably make this optimization [1]. In this case, the program can easily release some memory by clearing its cache.
You could also be running a program with garbage collection that normally wouldn't bother doing a full sweep until it hit some memory usage threshold; again, it can do this on request.
I'm sure that people can come up with other examples.
[1] In fact, a program can request that the kernel not cache its file-system requests, in which case it would have actual reason to cache what it thinks it might need again.