I learned that panels help with pacing. Very small panels tend to be read as fast while large/long panels are read slow. When introducing a new setting, I know of two ways. You can use long panels (which will slow down the pacing) or you can focus on specific elements of the environment with short/small panels (which will speed up the pacing).
Another thing I noticed about panels is that it can affect the mood of your comic. Using small panels and drawing characters large creates a claustrophobic feeling, which increases the idea of anxiety. Drawing a small character in a large panel can (sometimes) create a sense of loneliness, or insignificance (This one depends on what's going on for the page :/)
As a creator, I enjoy using long panels to slow down the pace so I can pick up the pace on the next page. As a reader, sometimes the comic uses long panels for just landscapes and I skim over those, which can be annoying.
(BTW A great book I recommend that talks A LOT about this is "Making Comics:Storytelling Secrets of Comic, Manga and Graphic Novels" by Scott McCloud.)