There is no difference at all for the general user. There are technical differences between them but they should not be relevant for the average user. The reason they make both types is that even in this day and age, there is equipment out there that will only play and record on one or the other format only.
I assume one company own the licence for the -R and another for the +R and I guess the person who owns the license for that technology gets paid a certain amount for every DVD that is made.
So if you are the company that owns the -R license you are going to continue to make -R disks, and the same will be true for whoever owns the +R license.