"The shower is in the bathroom."
Translation:Το ντους είναι στην τουαλέτα.
That would be like saying..."The shower is inside the bathroom." which of course would only make sense if we wanted to distinguish it from ... The show is not outside the bathroom."
That's the literal translation but "στην" already implies that it's in the bathroom so we usually don't use both words unless we want to emphasize.
To me it feels like στο/στην often don't need the additional modifiers, but they're used anyway. Is there some general rule in when they're needed and when not?
Indeed, "στον/στη(ν)/στο..." are often used on their own, but I wouldn't say there's such a thing like a rule, it's more like common sense: If one does not need to/want to be that precise considering the position of the thing in discussion (like jaye16 has pointed out above), then using "στον" etc on its own does the job ;)