iOS lets you restrict apps from using cellular data, so it only makes sense to let users do the same with Wi-Fi networks. Although cellular data is more prone to being limited by caps and speed reductions, Wi-Fi networks can sometimes cause issues with apps because of how they're configured or raise security concerns.
By installing a new jailbreak tweak called ConditionalWiFi2 (iOS 10) by iOS developer Creatix, you can choose what apps can connect to the internet over a Wi-Fi connection.