A comic story is divided into pages, and those pages are divided into panels. There aren't a lot of other media which have this natural rhythm built into them. Film, for instance, is not divided in this way. Television's only unit is marked by the commercial breaks, and even those are variable. It can be argued that prose fiction is divided by pages, but this is very rarely used in any way by the writer.
Novels flow from one page to the next without any regard to where the page breaks fall.
This isn't true in comics. As you know, the page breaks are built right into the script. So let's take a look at the structure of a comic page. I should note here that I am a long way from understanding the intricacies of page design. This isn't meant to be a definitive guide in any way.