I've searched high and low and can't find an explanation to show what is the difference. Is 'ever' for emphasis here?
"Ever" means "at any time", so it does add meaning to the statement. However, there are only some situations where the context requires that you qualify what you mean by including "ever", so I can understand why you might think it sometimes unnecessary and only there for emphasis.
If you were talking about one single thing that you either have or have not seen, then it is unnecessary, for example:
Have you seen the movie Good Will Hunting?
The answer to this question can only really be yes or no, so there is no real need to qualify it with the word "ever". Idiomatically though, some people might say "have you ever seen the movie...", but it really adds no meaning to the sentence and this is an example of it being used just for emphasis.
However, there are some situations in which it goes without saying that you have seen something/someone at some time, and so adding "ever" would be more meaningful, for example:
Have you seen the mailman?
Have you ever seen the mailman?
The first would be interpreted as asking if you had seen the mailman this morning because it is likely you have seen him at some time; whereas the second example specifically asks if you have ever seen him at any time.
Ever means 'at any time'.
“Have you seen?” is said when you are asking someone if they have seen a particular person or thing in a specified short period of time maybe this hour or this day.
While, “have you ever seen?” is spoken when you are asking someone if they have seen a particular person or thing at any time maybe even once in their whole life.
• Have you seen Claire? I thought she was waiting for me at the bus stop.
• Have you ever seen a flying UFO? I thought they only exist in fiction.
• Have you seen my dog? I am looking for him for the past hour!
• Have you ever seen my dog? He's really cute!