When Im active on chemistry.SE, I use this feature, which allows you to typeset chemical formulas into the latex framework, a lot. And I can imagine quite a few circumstances where it could be handy e.g. in this answer. Chemical-Engineering is, of course, the field which will benefit most of this.
You can see an example of mhchem in action here, where I explained why a gummy bear burns away if in contact with heated potassium chlorate.