Fan service is a common term that gets thrown around whenever people want to criticize a moment in movies or TV for showing a nostalgic moment purely for the moment, so fans can remember what came before. It's a criticism that argues that the moment is just a cheap form of pandering, and is done without any understanding of what made the original moment great. And it's a term that people have used to describe and discredit the Star Wars series Ahsoka, especially Episode 5.
Source：https://www.cbr.com/ahsoka-fan-service-great/
