Why is the word "Pockets" used when referring to certain sections of places in a large area

The etymology of pocket is of a bag or pouch.

English word pocket comes from Proto-Germanic *puk-, Proto-Indo-European *bʰew-, and later Proto-Germanic *pukô (Bag; pouch.)

Etymologeek

This gives us the concept of an identifiable volume that may be separate (Scottish “poke”, as of a bag of sweets) or attached (trouser pocket). Hence, one of the contemporary definitions relevant to your question:

Pocket =

a group, area, or mass of something that is separate and different from what surrounds it:

”Among the staff there are some pockets of resistance to the planned changes” (= some small groups of them are opposed).

”The pilot said that we were going to encounter a pocket of turbulence (= an area of violently moving air).”

Cambridge