actually 3 purposes, water drains a lot better in a burlap sac than a bagGrow bags work well when planting in boulders. The bags will sit better among the rocks. I did a few of mine last year on a rocky slope and used contractor garbage bags inside of burlap sacs... The burlap serves 2 purposes: it looks more natural than plastic and it shields the roots from the heat that would build up from direct sun on black bags. Roots hate heat! The rocks where I had mine were dark grey so I took a sample rock and bought matching spray paint to paint the burlap so it would blend better. Hope this helps
good point im hoping to start within the next month getting everything prepared. your screen name sounds like your from the kootenayWell I had garbage bags inside the burlap but had holes in the bottom of the plastic bags. I found that with burlap alone my watering needs doubled so putting the plastic inside the burlap allowed for better water retention
thats about 5 hours from where i am lolOkanagan valleys about 4 hours from kootenays... Much hotter and alot less rain around here so even tho relatively close it's a completely different climate.