In my opinion, it is to prove a point -
Everyone thinks the earth is round, but an argument does exist for a flat earth that can't be easily disproved. It is arguing for the basis to argue to show that not everything is as cut and dry as things appear. Photos CAN BE manipulated, the government CAN BE in a conspiracy - are these strong arguments? No, but they do go a long way in disproving RE.
I try to bring up points that falsify FE that do not fit the models. This website is debating on the fact that there is no absoulte truth.
I am sure some people do believe in a flat earth here, but for the most part, the FE'ers are people who like to debate and don't take things at face value, and laugh at how pissed off people get when they can't make them believe the earth is round.