Tone user here 
They serve the same purpose like coloring or greyscale does, balancing panels, off setting foreground from background, add texture, help the story telling ect.
It’s just up to the taste of the artist if they like to use it or rather go with other techniques
Personally speaking I find tones a lot more appealing than grayscale, don’t ask my why but the overall look is just prettier to me. It always reminds of old manga I used to read as a kid
Besides that, I’m a traditional artist and love to hold a finished piece of art in my hands once I’m done.
For me tones are a safer alternative to add a finishing touch to my comic than greying areas out with washes or markers.
If you screw up with tones you can peel them off and start over again. While correcting mistakes done with brush and ink is a lot more difficult.
Only downsides tones are pricey and quite time consuming. (But so is coloring too, right?
)