Generally the idea is that regions of memory that aren't being used are swapped out to disk, then other more useful things are brought into memory.
For example: you have a background program that you're not using and in the foreground you're crunching data from disk. In this scenario the most efficient use of your memory is to swap the background program out and pull the disk data into RAM.
For example: you have a background program that you're not using and in the foreground you're crunching data from disk. In this scenario the most efficient use of your memory is to swap the background program out and pull the disk data into RAM.