It's for backup. I do have a (completely full) Dropbox and files in a few other places. All of my photos are on Flickr, etc, and though I also have them stored on my laptop the recent changes Fickr made did just serve to remind me that the cloud services we rely on are subject to so many things beyond our control that I just want a second, locally backed up option. Russell had an old I think 500GB external hard drive, but I thought I'd get myself something new only because I've had a backup hard drive fail on me before that had been given to me and I could never get my stuff back off it as it corrupted loads of my files, and it was at the same time I never had the laptop I'd been working on, and I lost a heap of files and work. I think that's why I'd initially thought of the solid state as I thought it would be less liable to corrupting files in storage. My Macbook is six years old (today! Happy birthday, Macbook!) and as my last one actually exploded...
If I could get a second hand one that I wasn't nervous about losing things on (I know there are no guarantees, and it was just bad luck/circumstance last time) then again I am absolutely going to take it that there are people here that know far, far better than me.
It says I'm using about 350GB of the storage, but a lot of that will be system files, so it doesn't have to be hugely huge