Think of the root file-system (a.k.a "/") as like a C: drive on Windows. Its full of stuff that you may never need to touch. Installed apps typically go into /opt which is like "C:/Program Files", but stored in another partition.

Unused RAM is a waste of power and money. More RAM is in use than you think. Conky shows free RAM including RAM that is allocated to buffers and cached files, as that can be freed up for apps to use.