One more note on this; backups aren't backups until they've been tested. If you back up to an external hard drive every day and find out a year later that nothing actually got written to it because your backup software was turned off by someone else, the drive was unplugged, the drive appeared to work but never actually saved the data to the disk, etc., you have nothing. Regularly tested backups are important, not just backups in general.

This isn't as critical for basic stuff like miscellaneous old files, but for bitcoin wallets, work projects and the like it's a good idea to check on it every once in a while to make sure it's still working properly. On the other hand, I lost my old bitcoin wallet (and every trace of the address, I don't think I had much if any in there but I'm not certain and I can't find it to check) from a few years ago when deleting an old backup that I didn't think I needed anymore, so you may want to hang on the the other stuff too. Storage is cheap, ~$100 every year or two is worth it to make sure you never lose anything that you might want later.