What a great question, Reyginus. That's good advice from
@NickB too, and there are a few more you could add to that list, but those three from what I've seen are about the best, along with Afternic which charges more but has a wider spread of prospective buyers. There is no charge for listing with any of them but all charge a commission once a sale is made.
But that doesn't answer your question, "Why?", which is more difficult to do in a sentence or two. I'll put it probably a bit too simply, so just ask and I'll try to fill in any gaps.
A potential buyer needs to find your domain name. Unless you have something very special, like a one common word, single syllable, dotcom which appears in many searches all your domain name can do is appear in a list and that only if it is already listed somewhere. Even a very good name, such as caninepet dot com, two easily understood, easily spelt words which pass the radio test, would be difficult to find a buyer for. An equally valuable but not dictionary word such as mikalu dot com, not in the dictionary and more suitable as a brand than as a marketing campaign name would be virtually impossible.
A landing page on such domain names gives each a reasonable possibility of turning up on a web search, thus helping potential buyers to find you.
The second part of your question is more open to interpretation of what's important. Take a look at the lists on Epik, Dan, or any of the others. Find the landing pages of domains on each. Typing a few of the domains into your address bar should take you to their landing pages. By using more than one site you can see what each emphasises. Epik use one approach and the users can vary that within the available parameters. And so with each of the others.