Online IDEs remove a large part of the hassle of having to set up a plethora of tools even before any coding can begin, they can be accessed from any computer, can be easily shared and make it easier for everyone contributing to have the same setup. Online IDEs allow building, testing and previewing projects in the cloud.
The obvious drawback to online IDEs is Internet dependency. They can quickly become unreliable if there's any chance of Internet outages. And while whole environments can be spun up to test and preview projects, a cloud IDE is not a host. Projects can't be stored in the cloud and sharing has limits.