For most of us, they easily soak up far more time than they save, capturing our attention dozens of times daily, and directing it to gratifying but mostly forgettable activities, usually infused with advertising.
I empathize. I just set up a new phone and was frustrated by all the unwanted notifications spammed at me from my OS and the latest versions of so many of my apps. It seems a lot worse than the last time I did this few years ago, and I'm constantly tweaking settings to stomp them out.
This began as a surgical exercise - usually I don't want to kill all the functionality, just turn off the specific alerts I don't care about. But it's gotten to the point I feel like I'm waging war against an ecosystem of developers intent to squander my attention without recognizing it's an incredibly scare and valuable resource.
Messenger in particular was so difficult to get to shut up that I just wound up removing all Facebook's apps. They're not the only offender. If you have an Android, go into App Info for Google Maps (edit: then tap Notifications) and look at the sheer volume of stuff it wants to nag you about.