I'll admit I haven't read the books, but I've seen in subtitles and other things that knights in Game of Thrones are addressed as "Ser".
It seems to me that fantasy works like this have similarities to medieval Europe and have aristocracies, knights, kings, queens, lords, ladies and so on.
The Kings, Queens and Lords in Game of Thrones use the usual real-life counterparts, so why is "Ser" an outlier when knights in England are addressed as "Sir"?
The answer given on the Movies and TV site could be useful to you.
Basically, George RR Martin invented "Ser" to add some difference between his novels and the real world.
To look at this from a more general writing perspective it enables the author to convey the "strangeness" of their world without compromising the readers ability to understand what they are conveying.
You know that in English the same title is "Sir" so when you see "Ser" it clearly tells you that you aren't in the world you know but the similarity means that you don't need any exposition from the author to tell you what the title means.
When there's a large amount of worldbuilding going on (Dragons, wights, etc) you're going to have to do a certain amount of exposition but where you can do something that's self-evident all the better. Especially given the books are written in third-person limited and there's no audience surrogate it helps you avoid situations where characters have to think about something that they wouldn't naturally do in order to explain it to the reader.
