The RAM is where applications are loaded when running - so if you have a few apps open, then they are loaded into the RAM.
If it needs extra space, it uses 'virtual ram', which comes from the hard drive, and is significantly slower.
When minimised, if space is tight then the app is moved to the virtual ram. When you open that app again to full screen, it has to copy that data from the hard drive into the main system RAM.
The less this process is required, the quicker switching between apps will be....
Did that make sense?