1) Allows me to upgrade to the latest OS X Sierra.
2) Is faster
3) Gives me more space
This is not as easy as with any other desktop PC... First you have to remove the glass.
You also have to undo 4 cables and you need to be gentle.
So I'm putting in a 2TB SSD/HDD hybrid. A bit more space and a bit more speed but most important I can use the latest OS again.