A couple weeks ago, I got a warning from AT&T that I had used almost half of my monthly data allowance.  I couldn't figure out why that would be.  I hadn't been away from wifi any more often than normal.  I wasn't streaming video across my mobile.  It didn't make sense.

Then a friend shared this status, and it WAS what had happened on my phone.  I recommend making the suggested change.  It made a big difference in my data usage.