1. Yes you need TPMS in your car.
2. If ever your tire pressure is low or your tire is flat, you won't easily know. TPMS monitor will notify you immediately, possible avoiding any accidents.
3. If there are no TPMS sensors in the car, you'll get a warning light every time you start the engine. This happens even if one of the four sensors is malfunctioning/missing.
4. You can definitely use your existing TPMS sensors with aftermarket wheels. Make sure you check with the wheel manufacturer. I have aftermarket wheels and I use my stock TPMS with them.
5. ...and finally, TPMS cannot be disabled.