About the unnecessity of prologues

Most of them, that is. I dislike those that were just added for the sake of it. Why would people do that? Prologues only have a place when they're relevant, for instance, when, in Fantasy or Science Fiction, characters of the here and now will appear in future and what they are discovering, the quarrels or change to the landscape they're in are important to the storyline which takes place either centuries or years ahead. 

Or, if it sets the scene for the whole book, referring to significant changes to the world the characters live in. That I will read with interest.

But too many times, prologues are either the back story of characters I don't even know yet, hence not really care about, or they are an excerpt of the novel, which spoils the plot for me. I want to read it and find out for myself.

In many cases, prologues can be avoided and easily interwoven in the actual book. Let me give you an example: I'm currently reading a book where the prologue shows a female dancer on the height of her career, just proposed by her long-term (extremely unlikable, arsehole) boyfriend, in a car, on the way to her parents, in a thunderstorm. They have an accident.

The actual story starts seven years later, she's not with that guy anymore and not an active dancer, but a dance teacher. The author is regularly referring to the past, to her ex, to the accident as it's the cause for her not being able to dance without pain anymore.
I keep wondering what purpose this prologue served. Those few pages could have easily been combined into two sentences of back story when we actually know more about that woman and how much dancing means to her, and the new man in her life, for that matter. If the author wrote, she danced with pain, the reader wonders why, and there she could've mentioned the accident. Simple and most effective.

And I read plenty of prologues like that. Redundant, in my opinion. If you need to write a prologue, do it because it would enhance the novel, needs to be there. Not because you like the word 'prologue' so much.