None, given that it essentially runs in a browser anyway (Electron). I suppose it's very useful if you have ~200MB of RAM and ~80MB disk space that you really want to get rid of.
WPF/UWP instead of WinForms and I agree. I don't even get why so many people like writing web apps, I feel a hell of a lot more comfortable working with a proper widget toolkit (my preference being GTK or WPF) without the constraints enforced on me by a browser and JavaScript.
It's because there are a lot of people here obsessed with reaching the largest possible audience they possibly can. And since every brother and their mother are on mobile, or, at least, a web browser...
The way I see it, mainstream masses are finally (FINALLY!) leaving the world of desktop computing behind, so now it can return to the nerds and people who just wanna do shit
I think that web app benefits vastly outweigh the constraints. Distribution is easy (give people the URL), the app is always up to date, and it's cross platform. Unless you absolutely need the features and/or speed of a device, you should seriously consider a web app vs. a not web app.
With the recent popularity of Electron, I wonder why less attention has been given to Qt Quick. The JSON-like syntax of QML as well as JavaScript should be very familiar to web developers and you get software that looks much more "native" than what is achievable in the DOM (well, without reasonable effort).
I guess at the very least it'd be recognized as a separate window/app by the OS window manager, which might be convenient if you like to keep it open all the time and alt-tab to/from it.
However, at least on OS X, you can just use something like Fluid if that's all you want (http://fluidapp.com/).
A browser is like a buggy mini-operating system that hogs resources, tries to replace native applications like PDF readers, word processors, and email clients, and uses JavaScript as its system language. Client applications circumvent the hell of modern web browsers. The benefit is not needing to use a web browser.
Wish there was some kind of a "Electron kitchen" where you could easily make your own app stating which website you want to use in this isolated way, without having to learn about Electron.
I hate Electron. :-|