@hrisskar it's a scheme to attack the credibility of science and education.

He wants people to debate & challenge him - he will refuse in the face of all evidence to concede that the Earth is round, and he will belittle and insult his opponent who claims it is. Eventually his opponent finds something to do that's more fun than talking in circles while being constantly insulted, and the flat-earther will claim victory when his opponent no longer contests the issue.

Now that he's "won" his debate, the flat-earther can attack his opponent in other ways. There's a number of methods by which he can go about doing so:
- he might imply his opponent's education was worthless since he "lost" the debate
- he might seize on simplifications, take comments out of context, or otherwise draw quotes from the debate in bad faith
- he might simply leave his opponent unwilling to challenge him on other subjects of greater controversy.

The flat-earthers tend to be aligned with the political right. Their end goal is often discrediting fields associated with the political left: race and gender study, climate science, evolutionary biology, etc. Sometimes the scheme more broadly targets higher education in general, which is seen as left-aligned in North America.

Similar strategy is employed by religious (Ken Ham, known for debating Bill Nye on creationism) and political (Steven Crowder, known for his "change my mind" sign) advocates in debate. The way to evade this particular trap is to refuse to allow your opponent to frame the debate such that they "win" if they don't concede.