I don't want to debate people about a conclusion that we agree on. Don't play devil's advocate to advocate something you think is wrong. Don't bait me to make arguments that you already knew and could have said yourself.
Why would anyone want to do that?
First, because they're uncertain. They don't believe X. They didn't reach X as a conclusion. But they can't fully rule it out either. They want to argue in favor of X without declaring themselves pro-X. They may think X is probably false, but they can't reach a clear conclusion with their current knowledge.
That's fine if you’re open about what you’re doing. If you have genuine questions and doubts, we can talk about it. Arguments for a position that you can't refute, but also haven't accepted, are fine. You don’t have to have reached a conclusion to argue for it; it’s also fine if you don’t have a conclusion yet (in which case we don’t already agree – in the sense of sharing the same conclusion).
If you're certain about your position, and we agree, then don't try to debate me. To debate, you either need to have stuff to learn or disagree with me. If you want to collaborate to develop the position further, that’s a discussion not a debate.
Why would someone want to debate if they agree with me and aren't trying to learn anything? Why play devil's advocate in that case?
A typical reason is they want me to tell them what to say to win debates with other people. They are acting as proxies for people who won't debate me so they can say my arguments to those people. Please don't. I want to debate with people who are being genuine. If the people who believe something aren't willing to debate me themselves, and you're convinced that something is wrong, don't try to debate me on their behalf in order to get me to say criticisms of people who don't want my criticism. Also, people sometimes mix this with plagiarism (using my arguments that they asked me for, but without giving me appropriate credit) which is even worse.
If you can’t win a debate yourself, you should reconsider your conclusion instead of trying to get me to win it for you. Based on your own knowledge, should you have reached the conclusion you did? Or should you have a more tentative conclusion that admits to some ignorance, confusion, etc.?
If you want help debating better – generally or for a particular topic – ask for that openly and directly. Ask for me to teach or help you in some way. Don’t pretend we’re debating while you’re actually trying to get a different kind of thing from me.
What if you want to make some arguments, and if they work it was a debate, but if they suck then it was never a debate and you were just asking for help? That is common but dishonest. Please don’t.