The reason people generally have a popdoor instead of just leaving the people door open all day is mainly that no matter where you live there are sometimes days when there's a limit to how much weather you want coming indoors. The small popdoor keeps out a lot more sideways rain, etc than an open people-door does.
The reason people generally have a popdoor instead of just an opening is, as WW says, so you can close it at night to keep predators out. it is easy to believe you have a 100% predatorproof run; as you will see from a browse of the Predators and Pests section of this forum, it is a lot harder to actually HAVE a 100% predatorproof run and KEEP it that way every day of the year. (A lot of people who lose chickens had left the popdoor open overnight b/c they were SURE nothing could POSSIBLY get in...) It's up to you, of course, just letting you know the deal.
Some people have got their chickens using hard plastic dog doors, but others have had trouble and given up. Flexible materials like translucent vinyl shelf liner or wide strips of canvas etc are a bit easier to train them to use. But of course not even remotely predatorproof, and some weather still blows in.
Hope this answers your question sorta, good luck, have fun,
Pat