RAM should be filled with 1) Things that are likely to be reused or 2) Things you want faster access to, or at least a constant fast access time when needed.

RAM should not be filled with 1) Things you will never use again (for example log files, or things that are very quickly invalidated by synchronous writes but rarely read) and 2) bloated data structures. For example if you have data format a and data format b that achieve the same thing, but b takes 30% more space at the same speed, b is very bad and should not be used.