Donk bets force your opponent to make mistakes.
I use it sometimes in HU SNGs at the Ultra Turbo pace (2 min blinds on AP) - for two reasons.
1. Set a precedence. When the blinds are still low, I can donk-bet to see how an opponent will react. Will they overbet on a raise? Will they fold, or cautiously call? After doing it a few times in small pots for small bets, the villain becomes accustomed to the move and mistakenly puts you on a hand - either good or bad - and reacts accordingly. Once the blinds go up, you can use this knowledge to your advantage. TpTk on a dry board in a medium sized pot? I donk-bet knowing that villain will raise with hands I've seen him raise with before, and am happy to repop him.
2. It throws your opponents off. When this move is used against me, I'll raise with just about any holdings I could justify calling with - and ussually talk shit in the process. I have AJo, the flop comes Q74, you bet 30T into a pot of 300T, I'll say something like "If you want to bet, bet like a man." And pop it up to 210T or so - and react accordingly. Also, donk bets are a great way to get your opponent to throw away their blinds when they are short stacked and have a poor M.
A lot of opponents are willing to fold blind after blind after blind after blind to min raises and min bets. And I'm happy to collect them. I always know where I'm at in a hand (I guess it comes with a lot of hands of poker, both live and online, and I'm rarely surprised when the hole cards get shown and my opponent had a hidden monster...) so I can use them to my advantage when it's HU and my opponent's stack is 4 BBs and I know they can't call another BB when it's half the pot, for instance.
A lot of hands are fit or fold when you're short-stacked, and a lot of flops miss your villain -
In a game format that isn't UT HU SnG, I won't use this move. I will lower the size of my bluffs - and CBets, because like I said, it throws your opponent off. "Why's he betting 80 into a pot of 400?" - yes, this is a donk bet, but my turn bet is going to be 400 - telling the story that I've had them the whole time.
Which adds value to my next donk bet, as the donk bet on the flop holds the threat of a bigger bet on the turn.
It's an interesting move, but it has limited application. If you want to post a blocking bet, size it appropriately. I very rarely don't raise a donk bet if my stack and hand can support it. If my hand has no business being in the pot, away it goes.