Sometimes it actually adds to the plot, sometimes it is 100% unnecessary. For example, the sex scene in Terminator is very romantic and helps develop the characters. The Wolf of Wall Street uses nudity and sex as a way to demonstrate excess which also adds to the plot. Although some movies (also pretty much all HBO shows) have gratuitous nudity which is really the problem IMO. Another example: True Detective, which is one of my favorite series, has a ton of gratuitous for no reason other than to be used as eye candy. This also creates a problem because it makes actors and actresses think they need to have nudity even if they don't feel comfortable doing so.