Crafting is a way to earn gil that you later use to buy things. Some quests like the relic at certain points require gil to buy items, and also to obtain materia, buying it is one option. But gil is not very important in the game and there are other ways to earn gil. Usually leveling a craft or gearing a crafting class needs you to spend gil (or a lot of gil) that you will recover in the future.

Being able to repair your gear everywhere is also very practical.

Being able to craft gear or food or housing items without buying it is also practical and to some people make them feel good. For example to level a new class you can craft gear you need.

You don't need to level a crafter, and it takes time too. It's another thing you can do. I started to craft while I waited the queues leveling the THM for swiftcast cross-class skill.