What are full-panel maternity pants/shorts?

Elle • Snap: MochiBunn
This is probably a really dumb question but... I'm confused looking at maternity pants. 
What is the "panel" for? Do your pants fall off? Is it to keep your belly covered if your shirt goes up? 
I'm confused- it's really hot in Arizona and I'm not sure I want/need that extra coverage. 
What am I missing here? 
Thanks! Feel free to tease me for not having a clue! 💕😉

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