When a front-end (HTML) generation is run, the project switchs to Dirty Mode. This means that some server side features may not be available, and HTML files could try to access them, generating client-side errors.
Why should I work in Dirty Mode? The answer is the same if we ask "Why should I Generate HTML files only?"
When developers are trying out changes in their application model (e.g. prototyping screens), they may choose to avoid full generation process (which implies time-consuming Java classes generation, compilation, and context refresh), so as to get a quick preview of the introduced changes.