If you ever go online using mobile connection, and if you have limited data there, then TripMode is a no brainer for you.
It does two things really well.
First, it tracks how much data each app (or process) consumes. It tracks Safari, Mail, Spotify - but also things like iCloud, Photos syncing and other seemingly innocent processes.
Processes like iCloud or Photos sync can easily burn through gigabytes of data in a matter of minutes, thanks to fast LTE transfer speeds we have these days. Unfortunately, "gigabytes of data" is often our monthly allowance.
Second, it lets you disable Internet access for certain apps with a simple click. You just check or uncheck boxes, it will take care of the rest.
TripMode is just $7.99 and there is also a Windows version. (Sold separately.)