Thats when you need more RAM, and the installed RAM is not enough. Why don't you do a little experiment and try to stress the RAM and see how far 8GB takes you? launch applications and load tabs and watch that memory pressure gauge. Report back here.
btw, I just noticed that in your image that you think "swap" is the RAM. Where it says MEMORY
that is the RAM you see its no where near full. SWAP is when the computer loads information into RAM, when it doesn't need it, it stores it on the hard drive. When it needs it again, it calls it back into RAM. Don't worry about it, it all works magically.
EDIT: Just forget it,
@MrGunnyPT has posted
this video that shows that 8GB will take you very very far.