Yes, I should be using GIMP/Krita/Other community FOSS solution, but gosh darnit, sometimes I just wanna make a 20-second markup on a screenshot, and I wanna use the friggin' program that I have a 20-year muscle memory for!
Is it a perfect recreation? Of course not.
Will it do every sort of image editing I might want? A definite no.
Does everybody either quickly learn it or already have its interface burned into their memory? Yep.
Does it work well enough to do a quick edit on pretty much any OS (including mobile) without needing anything but a browser and an easy-to-remember link? YES.
If we are being honest that's probably an implementation thing, and not just a function of it being JS, but once gain performance problems in a JS app damage the experience for me.
1. Screenshot thing with screenshot rectangle selector (Win+S on OneNote 2013)
2. Draw red circles and arrows all over it
3. Repeat #1 on mspaint canvas
4. Paste screenshot with annotations to whatever I need pasting it to.
The whole thing probably takes less than a couple of megabytes.