I think for most of the markets where people can influence the outcome, those are mostly just for fun.

For the more serious markets, like who will win the presidential election or sports games, both sides are incentivized enough to win that the existence of a betting market shouldn't influence the outcome, unless one side chooses to "throw" the contest. Thus, it does seem that in the more serious markets, especially sports betting, the prospect of one side losing on purpose is the much bigger concern than any other incentives effects towards encouraging one side towards winning.