Sometimes I use a donk bet with a questionable card to find out where I am. Sometimes it's better than answering the opponent's continued bet
This is definitely not the right reason for betting. Youre either betting to make worse hands call (value bet) or betting to force stronger hands fold (bluff). Betting cuz "why not" definitely isnt the reason that good player would use for his bets.
On topic, while i was grinding 10NL (havent played for more than a year so dont judge me if im bit rusty) i had stat on my hud called "donk fold" its the stat showing how often a player who donked, folded to a raise, dont be suprised to see that that number is really hight on average fish, i used to exploit that stat a lot back then...
On toppic bit closer, i really dont see the good reason to donk beside to appear fish to someone new at the table when youre strong and love to get raised, beside that vs regs, only thing that youre actually folding are weak hands, made hands or draws will call or raise if better, then you have to reevaluate turn if called and have to invest more money into already big pot, it is just wasting money IMO, its better to go into check call mode ahdn keep his range wide and possibly have some showdown value till the end or improve by the river...