Well for one, security… if your ebike has only a button power switch, then anyone could turn it on and ride it away quickly(unless you had a lock, but still), or intentionally waste battery, but if it had a key switch, someone would not be able to turn it on without the key, and even if it was unlocked it would be harder to steal or mess with. And adding an external key switch is, of course an option, but then you’d need to carry another key, which could be a hassle. Also, it would reduce the need for custom control boards to turn the battery on and off, helping to standardize parts!