You can still bet on near certain events / events in progress, but there’s not necessarily a benefit in doing so as the odds shift. If something is believed to have a 50% chance of occurring then theoretically the bet would cost 0.5 for a payout of 1 (of you win). As the outcome becomes more (or less) likely, the cost of the bet changes to reflect that. In a prediction market, it’s similar to stock market in that in order for you to buy a share / place a prediction bet, someone has to be selling a share/taking the other side of the bet and the prices shift based on perceived value of the underlying thing being traded (stocks or predictions).