Actually it is simple Dpi is probably the digital simulation of a film. Like the quality of negative on how many light dots it will have, in combination with the shutter opening that throws light dots it could be compared with your Pixel size. the smaller the shutter the sharper your Image ( Negative ) the more small dots good for depth focus if you have something right in front of your camera. Shutter open will only keep your focus sharp and background blurry equals large light dots. So in a way all this is to be able to digitalize mechanic shots! The Function of Printing is similar , Obviously !  Just that you need these Digital settings to get the right amounts of sharp dots for your Printing if you want a good quality Picture. If you google you will see what amount of  DPI it will need to have a negative that can be enlarged to a poster without any loss. 

Ever been in a Movie theater ? you need to imagine a little square image that has such a high compression that the image projected on the huge screen remains totally sharp.

I do not believe that Dpi has been developed just as a fun setting that keeps your Images always the same no matter how high you set it up :)  and that just because Poser 12 can't handle it anymore that all the descriptions in the net about dpi are false statements .