Why does DVD + R/RW exist ?

[mud ver.2]

I know the difference between -R & +R is the format. I know some players/burners support one, or both. But WHY ?

Why are there even two types of media ?? Will DVD -R/RW be fazed out by the industry ? Is +R/RW becoming a new standard ? Why the need for both !?

I don't want to invest alot of money into backing up DVD's if the whole system will be obsolete next year !