A few years ago I was playing with hidden files (yes, that was a dumb move) and I accidentally deleted an important symlink.
The result: My mac didn't want to boot anymore. I let you image how freaked out I was. A lot is an understatement.
Luckily I had a 1 week old backup on my external HDD and booted from it. I dived into the finder and found what I deleted (just one single file) and restored it. Rebooted and everything was fine. If I didn't have it I would have been forced to either do a clean restore or try to figure it out manually and it would have been a huge inconvenience. Basically, at least one full day wasted, probably more.
Most people get a backup after they experience a catastrophic loss. Don't wait until that point!
The barrier to entry is pretty low nowadays. Black blaze for example offers off-site backups for 5$ a month, or you can buy your own HDD for relatively cheap (this post is in no way sponsored, I just use them and they worked well for me). It's surely worth more than the data you are eventually going to lose if you decide not to back up (yes, a decision not to do anything is still a decision).