Well SLI uses the graphic capacity of your two graphic card by creating a "Link" between those two. They do make game runs and look better( if setup proprely)! You have two graphic cards working on processing images that are normally processed by only one!
Unfortunately, not all hardwares works well with a SLI config. Some games tends to run like horse shit on SLI so sometime, you are just better with only one GPU. You need to find the perfect config for your computer!