"rain" (U) is simply water that falls from the sky, while "rains" are showers or episodes of rain. "Works" are single pieces of work. We say "road works" and "the works of Shakespeare".
I see those words 'rain' and 'works' appear quite a lot in their countable forms but rarely I see the other majority?? of uncountable nouns appear as countable nouns so readily and accepted with open understanding.
For example, I think, you will rarely or, in that matter, may never see the words like "nervousness" or "loveliness" appearing in countable forms, although