Having the Galaxy Apps store installed on you Galaxy S8 or S8+ alongside the Play Store isn't ideal, but it's the only way to update some of your phone's core services. What isn't necessary at all are its notifications, which can be useful sometimes but are often just promotional pushes you may not care about.
You can turn off the notifications for the app entirely, though, and it only takes a couple steps. Here's how to get it done.
How to turn off Galaxy Apps notifications.
Open Galaxy Apps from your home screen or app drawer.
Tap on the overflow menu button in the top-right corner.
Tap on Settings.
Tap on the toggle for Push notifications to turn off promotional notifications. These are the non-critical notifications about sales, deals and promotions from Samsung.
Tap on the toggle for Show updates ... to turn off app update notifications. You don't necessarily need these turned on so long as you have "Auto update apps" turned on above it.
With both of these toggles turned off, Galaxy Apps won't bother you at all anymore. So long as you keep the automatic updates turned on it will still update some core services periodically, and you won't ever have to open Galaxy Apps unless you want to.