[mhuffman]

The "grammar" part, at least in a professional setting. You might be shocked at how many people will write an email pretty much like they would talk to friends at a club or send a text message (complete with emojis!) or just generally butcher professional correspondence.

[dotancohen]

So it may be more attractive to employers to check their employees' output, rather than an individual checking his own?

[oneeyedpigeon]

No, it's also useful to check your own writing. I've used it as both an Editor and a Writer.