1. I always have my development environment. I can switch computers and be right back where I left off. Now instead of always carrying a laptop I just use whatever computer is at work or home. And I still use a laptop/chromebook for travelling.
2. Forced me to start using virtualized environments. I can setup and switch dev environments more easily. What takes hours of compiling open source programs on the Mac takes me a few minutes to setup. I can clone environments or setup environments more closely to what runs in production.
3. I can collaborate on code in real time very easily since I can share it just like a google doc.
4. I can setup dev environments for non-coders very easily. I needed my designer to do some HTML coding but running the full stack on her Mac would have taken hours to setup. Now she can help and not have to do as much setup she doesn't understand. Plus since everything is out in the cloud I can check up on her without needing access to her laptop.
With all that said I'm a web developer and the online IDE route works well for me. Other developers might not have the same experience. For example, a mobile developer will have a harder time getting everything working online for them.