iMessage notification banners not showing up on MacBook Air

I am very perplexed as to why iMessage notifications are not showing up on my MacBook Air as banners in the upper right hand side? I think they used to, but for some reason, it has stopped. I believe I have everything setup properly in settings. I can see a badge when I receive a message, but no banners. What does it mean by "show message preview" when unlocked vs. always?

I have my cell # registered for iMessage on all my iOS devices (iPhone and iPad and MacBookAir), as well as my email address. The messages show up no problem on the notifications screen on my phone. But, I only see a badge on iMessage (when not running) and I never see notifications even when iMessage is running.

Should I try a reset? I'm running OS high sierra 10.13.6

Also - what is the advantage of registering both a cell # and email address for contactable options in iMessage? Thanks.