It’s like cache, and useless data, system files, bits and pieces of apps and previous updates and other crap. People say it’s to make things smoother/faster, but that amount is stupid crazy. Definitely try resetting and restoring. It’s probably the long way to fix it, but I do it when it gets to 15-20gb, 70+ is insane, never seen that before lol hope it works for ya.

Holy crap. The way I do it when it gets bad, is not a very fun or easy way. Backup your phone, then fully reset the phone and restore from iCloud, that’ll bring system data way down. That’s a crazy amount of storage. Definitely need to reset and restore phone. It’ll probably take a little bit, but it’s worth it for that amount of storage. Geez.