Banner Notifications. Why?

I hate banner notifications. They are incredibly irritating. I find them completely pointless, and far more often than is reasonable they pop up right as I am about to tap an element at the top of the screen and so I accidentally tap the notification which switches the app. 99% of the time the notification isn’t even so important as to demand my immediate attention, and if something is important more often than not I will be expecting it and checking the relevant app regularly anyway.

So, I am not a UX designer, this is just a user’s perspective, but I’d like to know what the rationale for banner notifications is from a user experience perspective. Also, why do apps like facebook’s messenger and WhatsApp keep bugging me about re-enabling them? Do they think I gain anything from having them? Or do they stand to gain?

(Sorry I am not entirely sure how is best to tag this question)