I see two reasons to use a cloud IDE:

1) Device independent. If you want to start writing code on your work computer, work on it on your laptop when you get home, and debug it while at your mother-in-law's house, using a cloud IDE makes this much easier than having to sync files and have programs installed on all computers

2) Mobile. If you think of a breakthrough in your code when you only have your smartphone or tablet on you, being able to access it and work on it on devices that may not even support native development could be really useful.