I know that background traffic can be disabled but some apps do not obey that setting. For example Spotify downloaded over 500 MB this month and I have not even logged in. Actually I did not even open that app at all to see how much it actually downloads secretly.
These kind of apps are problem for those who have limited data plan. 500 MB per month for just one app is quite much and GlassWire statistics could help people to contact developers of those apps and ask for better behaving apps.