edit: spelling
When i'm debugging/developing/whatevering code that is going to run the same in all supported browsers, doing it in the editor directly allows me to switch tabs back and forth quicker and easier than having them as separate windows.
When i'm working on my laptop without any additional screens, being able to keep the view of the page on the same screen at the same time as the code is hugely beneficial.
When i'm at home I still tend to use a separate instance of the browser to "preview", but I still drop into the embedded one because the keybinding is so fast and easy to take a quick look then close the tab.
Plus being able to open bookmarks to random documentation websites and see the docs in the most up to date format in their "correct" formatting right in the editor is a huge bonus too.
Completely off-topic remark: I always find it funny that Web is hailed as the Write Once, Run Everywhere there is a browser, but then words "supported", or "compatible" start cropping up.
Really any more the only things that aren't "cross-platform" are bugs, and very new stuff.
That says more about you than it does about atom.
You should check out the IDEA line of IDEs sometimes.
I use it every day and not experienced what you are describing.
Because it can show you a preview of the page? Because, as far as code indexing is concerned, Atom doesn't do a very good job at all.
And as for code indexing, Atom doesn't do much of it at all out of the box. Everything is in plugins (by design). So it can use the full abilities of TypeScript or Flow if you add those plugins, or it can just use TernJS, or it can offer simple string-matching autocomplete.
It does as good of a job as what you plug into it does, as all it really does at the core is display text, and give a plugin interface.
Which was the point of them making it in a web browser. Leverage that as a platform to make extensions/plugins easier to write and maintain and allow them to do more.
Atom is a more capable "IDE" than any other IDE i've ever used, and it's easier to create and manage plugins for. Yeah, there are still some rough edges and things that I hate about it (honestly the whole "hanging for 30+ seconds because you accidentally opened a compiled file" shit really needs to get dealt with somehow, every release they chip a bit off of it, but it is still annoying), but it's by far the editor i'm the most productive with (by quite a large margin).