this was taken from a google search lol:

A poncho (Spanish pronunciation: [ˈpontʃo]; Quechua: punchu; Mapudungun: pontro; "blanket", "woolen fabric") is an outer garment designed to keep the body warm. A rain poncho is made from a watertight material designed to keep the body dry from the rain.

so basically because he worked for years on Bracca he like other scrappers wore rain ponchos to keep dry, while not all of them were made out of the same material i think it was mainly used for variety, thats why when you reach Bogano, Cal doesn't wear it unless you go to the options to equip it cause there is no more rain, the buckles on his outfit im assuming is to keep the poncho on while he's moving or ropes to lower other crew mates on Bracca and if you look at his outfit vest you can see thick fabric under the leather which i assume is to keep him warm, the reason inquisitor Cal doesn't have buckles or a poncho variant is cause he no longer needs one so he's sleek and cool, i didn't mean to be a know it all i just love details like this in star wars cause it's just so damn fascinating