treated wood

I guess I used the wrong forum location in Camping (tips, tricks) - here is copy - I will try to remove the other post painting treated wood? I have something to build with wood 2x4". I can obtain "treated" 2x4"...

I have something to build with wood 2x4". I can obtain "treated" 2x4" beams from Home Depot, these look different compared to the non-treated. The beams will be outdoors, direct sun and rain, ambient air temperature up to 40°C, termites. I wonder if and why "treated" wood needs to be painted...