Spare breaker spaces allow future development, such as including a workshop, hot tub, or EV battery charger

https://paxtontmaa197.inkharbory.com/posts/how-to-read-and-label-your-electrical-panel-like-a-pro

Extra breaker spaces enable future development, such as adding a workshop, jacuzzi, or EV battery charger, without changing the whole electrical panel needlessly.