I'm no expert but here's what i have learnt-ish of dnt
there are different types, i stay away from them.
someone more knowledgeable may reply but as i understand it most are a breach of auda rules by the current owner so they go and ask for it to expire so they don't P!$$ auda off, also it could be a complaint driven eg: a trademark issue, a business name issue, a competitor issue, "passing off" issue.
sometimes its bankruptcy
people do buy the "pd's" and mostly i see its people that do have a close relationship to the domain words, some just risk it.
you see a fair few pd's come back again and again as they keep getting auda calls to delete them.
pd's can also be a close of business, an expired ABN, i think auda try to clean up dead domains as well.
make sure you do some trademark searches, business name searches, way back when, who is etc thus create a picture of the domain before investing.
lastly, don't trust my advice, as i said, i don't buy pd's