Use it for whatever you consider necessary. I have several external drives I use for various types of backups.
The very basic use of a second drive is a backup of your data. Having all of it in only one place is a sure way to eventually lose it. All drives die at some point, including SSDs.
Don't know what size SSD you bought, but my suggestion would be to at least duplicate the Mac's main drive. Such as, if you have that split into two partitions (one for the startup drive, and the second for other data), then the external should have at least the same setup so you can make clones of both.
Then if at any point the internal drive fails, you can boot to the external and try to repair the main drive, or clone the backup to the main drive to get it going again.
You also must have a backup regimen. Creating a first cloned backup isn't much good if you never update again. Any new data won't be on the backup, such as newer emails and other personal documents and files.
You can also setup the entire thing for Time Machine and let it perform your backups on a regular basis automatically. There are various ways to use it, but a backup is, far and away, the most important. Unless, irretrievably losing all of your data at some point doesn't bother you.