Wi-Fi Assist is a new tool that lets iPhone users tap into cellular data when Wi-Fi signals are too weak. The problem is: It uses cellular data instead of Wi-Fi, thereby racking up big bills for some folks who are not on unlimited data plans.
Some users reported seeing their cellular data usage skyrocket after having the feature switched on. Plenty of sites are telling people how to switch it off by going to Settings->Cellular->scroll down to Wi-Fi Assist to turn it off. But Alex Hern of The Guardian shares another point of view: He suggests leaving it on if you want to continue with that streaming media from the home or office out into the wider world where it can pick up LTE. Just keep an eye on usage. Article