Donk bet, most of the time it's made by players who don't know how to play, for example, when they have a project on the flop, and they want to see a cheap turn, and they make mini donk bets.
But there are also experienced players who make donk bets.
They are regularly done when you are against a good player.
For example, if you think that this player can check the flop, then to avoid giving the turn, you could make a donk bet.
gl mate