It might be related to the Yeoman project, or they might have adopted this from somewhere else, but it seems to be the default for all of their generators. Aside from Yeoman generators I've also noticed it in other projects that have nothing to do with Yeoman (this one for example).
Where did this convention come from, and what are the reasons for using this over a
src folder, for example (which used to be more common)? Or is it just random preference?
p.s.: As far as I can tell it doesn't seem to be strictly related to it being a web-app instead of a web-site (which is rather subjective anyway).