I'm not meaning to be insulting here, but I want to explain this simply so everyone gets it:
- Disk is a resource.
- Memory is a resource.
- Memory is faster and consumes less power than disk.
- Any time the system can load something into memory for expected reuse it should.
Let's take a simple example. The OS wants a directory table, so it's powering up the hard drive (or SSD). It wants to grab, say, 32k. It's likely it'll want another 32k after that. So what's the energy cost for grabbing both the current request and the next at once? If there's not much energy cost, instead of getting 32k, why not get 32MB and keep it held lightly so that some other request purges it out? That saves spinning up the hardware, it means more performance, and memory might as well be used if it's there.
Don't worry about memory use. On a modern computer, the OS should be using lots of memory. Don't even worry about "low" memory. Worry about running out to the point it compromises performance.