From my understanding works like this
Every enemy has a set DAR.
For example:
lizards have (something like 35%).
gie gue has 80%.
At 100% room DAR the above % will be as they are. At 200% A lizard will have 70% and a gie gue will be 100%.
RDR i am unsure on the mathematics but I can certainly say it's a very good thing to rise.
Note: the HBR quests boost the room DAR for those quests (viewable via /partyinfo) which then gets placed into calculation with the enemies Base DAR etc.
Essentially it means alot more shit will drop, and among alot of shit a few possible nice things. HBR is great for general crap, pds, mats, dolls, items to sell, meseta. After building some % with a good group you will notice a huge difference in the amount of stuff dropping.