The watt meter is important because once you hit 10AH - 12 AH, time to stop using the battery. You'll likely never use that much but if you ever want to go joy riding through the neighborhoods, then you'll need a watt meter to observe how much battery you have used and know when it's time to go back home or stop using the battery.

I go joy riding on mine so when I've used about 5 - 6 AH, I know it's time to come home because it will take another 5 to 6 AH on the way back so when I get home, I've used about 10 AH which is 50 % of the battery.

Like I said, though, if you want to use more than 10 - 12 AH of the battery, you'll need a BMS for it. You'd only need a BMS if you wanted to go further than 20 miles though. Most people don't need to go that far anyway. That's a pretty long ride.