First, the tank gets waterlogged. It fills with air, then slowly gets lazy and fills with water instead. When you turn on the tap, you get a ridiculous sputter, not a steady stream. It’s like your pipes are trying to burp at you.
Second, you might need to fix a leaky faucet or replace a hose. If your tank is pressurized, you’ll get a face full of cold water. That’s a bad surprise. Nobody likes a cold shower from a basement tank.
Third, winter is coming (cue dramatic music). If you leave water in the tank when it freezes, it can crack like an egg in a microwave. Don’t do that. It’s a $500 mistake you can avoid in 20 minutes.