There are plenty of reasons why you'd want to train the skill. It gives you access to teleports around Gielinor that will save you a lot of time. It's 1 of 3 combat styles the will help you slay certain creatures and bosses effectively. Also, it gives you access to money-making methods, such as high alchemy. Finally, one of the main reasons you'd want to train Magic is to gain access to Prifddinas. It's a city that provides multiple effective methods of training skills, and Magic is a requirement for the main quest.