Let's start with some images.
Panniers are one of my favorite bags to make and here's why.
Panniers last forever. I mean they will last for your life span. So to you, relative to your life span, that's forever.
Nowadays, when considering a purchase, two things come to mind: it will last or it's temporary. Both are fine.
I think panniers are the easiest way to carry large amounts of stuff on a "typical" bike.
Bicycle racks are typically made of the same diameter tubing, so the majority of panniers will fit the majority of racks in the world. This is a good thing!