So let's say that you're a cat, and you live on a planet of mostly cats. When you turn on the TV there's a bunch of TV shows about cats doing cat stuff. The ads are for litterboxes and catnip. Lots of jokes about having nine lives.
Then suddenly you turn on the TV one day to watch a new show, and one of the side characters is a dog, who experiences the struggles you'd expect in a cat's world - barking is banned, playing fetch is considered distasteful, and going for walkies makes you a target for harassment. "Why does it have to be a dog?" you wonder. "I just want to see characters, I don't care about how hard it is to be a dog. Do dogs really care if they're up there or not? Why don't they just enjoy cat stories? Dogs and cats are both kinds of animals, how different can it be?" You start seeing ads for dog food and tennis balls, and just get pissed off because it's not relevant to you. And to be honest, some dogs probably agree with you, like, "I don't need a token dog on a show to feel like my existence is validated, and I already know where I can get my tennis balls." And that's fine too. But don't make the mistake of thinking that every member of an underrepresented group falls into the same line of thought.
Whenever you are a member of a class that is always represented, it's super easy to not realize how much that can impact the world, and how isolating it can feel to not see yourself represented. On the flip side, it means that when stories are presented that AREN'T your story, it sticks out to you like a sore thumb and can be really jarring. It doesn't have anything to do with you, so why are you looking at it?
But imagine that experience from the other side of the equation. Things aren't quite so stark now, but when I was growing up as a queer woman in a very tiny rural town, I was never exposed to a single example of a romantic relationship between women. I had a bunch of feelings, but no clue how to deal with them. Romantic comedies were full of jokes about how woman be shopping and eating ice cream when they're sad, and men be going to the gym. I had no cues, context, or examples of a healthy relationship between women to show me how it might work or demonstrate that it was even possible. Because the small world I inhabited had no room for a big part of my identity in it, and no way for me to model understanding that part of myself, I had to do a LOT of self-discovery and wrangle a lot of self-loathing. It's not about "wanting to see myself," it's about all of the stuff that seeing yourself DOES for you without you realizing it, especially when you're young and in the process of figuring out who you are.
A number of people are doing a decent job talking about the issue of representation in this thread. There are also much deeper issues at play than representation because whenever the world is built around a particular kind of thing being "normal", and only one type of person is represented in that world, sh*t gets really weird. That's when you get stuff like crash test dummies not accounting for a big variety of kinds of bodies, resulting in disproportionate fatalities in accidents. It's when you get phones that are too big for many women's hands, or automatic photo-sorting algorithms that categorize non-white people as animals.
TL;DR: When the world is literally designed for you, calling out issues for the people it isn't designed for can feel jarring and weird. But for many of the people it isn't designed for, it can make a huge difference.