While most users should and would never disable iMessage, there might be good reason to. The biggest reason is if you constantly switch between iOS and another mobile operating system constantly with your phone number.
iMessage lets you send and receive messages via data with your phone number. This means that you can send messages in areas where cellular service isn’t available, such as on a plane. The service lets you send and receive messages from your iPhone, iPad, Apple Watch, Mac, and even your HomePod with ease.