Do you know what info or services its auto syncs? Is it by chance the same thing as turning on or off "restrict background data" ??
I'm not 100% certain but my assumption would be that it disables every service that requires background data sync, including Gmail, play store, music, etc...
Yes I think it is the same as restrict background data, but easier to get to so you don't have to dive into settings to enable/disable.
I think it is handy if you are low on battery and need to stretch it out, or if on limited data plan.