This banner is a notification from the system. It appears every time you're connected via mobile data and you try to launch an app with mobile data connection restricted.
For example, I turned off cellular data for Dropbox on my phone. Now, imagine I don't have a wifi connection available but I need to open an important document stored at my Dropbox. I'd probably not even remember I had turned off Dropbox's cellular data access, since I use it mostly over wifi. I'd just launch the app as I do with the other ones.
Because of that, instead of displaying a broken application and to prevent me from getting frustrated trying to figure out why Dropbox won't connect, Apple created this banner. It remminds me I restricted that app, but if I need to, I can restore its access.
"But why does Spotify have this banner if it has an offline mode?" The answer lies on what you need to think about when building an Operating System.
With iOS, Apple needs to create something that covers the majority of the possible scenarios their custumers will face. In the case of "Restricting Cellular Data Access", almost all the apps people would restrict only work with an internet connection. Since the apps only work online, it makes sense to add a remminder to the user that the app was restricted if he/she try to use that app with mobile data. That's why Apple created this notification. That is what happens with 85% of the user base, I'd say.
On the rest of the scenarios, the app would either require no internet connection at all or have an offline mode (Spotify's case). This is what you're facing. You're on the minority of the cases.
You simply turned off data access to that app because you don't want to waste your data plan with something that has an offline mode! This app doesn't require to be connected in order to function, why do you need to see this annoying remminder banner everytime!?
First, let's understand when the banner appears. The conditions are:
- Connected to the internet with Cellular Data
- Launch an App
- The app's mobile data access is turned off
The majority of Apple's custumers would only face this scenario some times. However, the minority, who uses apps with offline mode, would trigger the banner on a daily basis or even more frequently.
As I explained before, Apple needs to solve the majority of the possible problems. They never meant to keep bugging you. They only want to prevent that user who forgot he restricted his Dropbox from having a bad experience with his phone by remminding him he turned off data for that app, instead of letting him assume his phone has some problem or Dropbox is full of bugs.
I hope this answered your question! ;D