I have seen an answer beginning with the prefix: "TL;DR". Did the person who answered it write their answer title that way? Or did the OP add that to the answer title? Or, are there other reasons I might see "TL;DR" as a prefix? I am detailed (verbose) in how I write answers and I would hate for one of my answers to be marked: "TL;DR'
Did the person who answered it write their answer title that way?
Yes, they did so. If you click on the 'edited' link shown under such posts, you'll see how the original revision looked like; here is an example.
Sometimes they help readers, just like abstracts of scientific papers: you can quickly see if the answer would contain the information you're looking for. But most Stack Exchange answers are short enough to be skimmed efficiently (at least for me) and their benefit is debatable. It's not required at all to have them, and I haven't seen a single instance where a users added a "TL;DR" to somebody else's answer.