It holds a hammer. That is the whole answer, and if you came here to settle a bet you can go collect.
The longer answer is more interesting, because nothing on a pair of carpenter jeans is decoration. Each strange detail is a fix for a specific annoyance, invented by people who worked on ladders and roofs and got tired of climbing back down for the thing they forgot.
The loop
A framing hammer weighs well over a pound and has nowhere to live. Put it in a back pocket and it tips out when you crouch. Set it on a joist and it ends up on someone's foot. The loop on the left leg lets the head hang at mid-thigh where your hand falls naturally, handle down, out of the way of your knee. You drop it in without looking and draw it the same way.
These days the loop carries plenty that isn't a hammer: a rag, a tape looped through by its clip, a pair of gloves, a small level, a dog leash. Painters hang a wet-edge rag there. Warehouse workers clip a scanner lanyard to it. It has stayed on the pants for a century because a fabric strap that costs nothing turns out to be useful to almost everyone.
The narrow pockets on the right leg
Those tall, skinny pockets stacked down the outside of the thigh were built for a folding wooden rule and a flat carpenter's pencil, which won't roll off a roof. Today they fit a utility knife, a Sharpie, a tire gauge, a phone if you don't mind it riding low. The point is the position: on the side of the leg, tools stay put when you kneel, and they aren't under you when you sit down in the truck.
The cut
Carpenter jeans run straight and roomy through the thigh for a reason that becomes obvious the first time you squat in a slim pair. You need slack in the seat to get down to a baseboard and back up forty times a day. The leg opening is wide enough to sit over a work boot without bunching.
What separates a real pair from a costume
Carpenter pants come around in fashion every few years, and when they do, the racks fill with lightweight versions that have the loop and none of the substance. Three checks tell you which kind you are holding.
Weight. Most work pants on the market land somewhere between 9 and 12.5 ounces per yard. Our CA8929 Carpenter Jean is cut from 14.5 oz 100% cotton. You feel the difference when you pick them up, and again two years later when the knees are still there.
Where the loop is anchored. A loop sewn into the side seam and the pocket edge will carry weight. One tacked onto the surface of the leg is an ornament.
The size run. A brand that means it for work makes it for working bodies. The CA8929 runs from a 28 to a 48 waist with inseams from 28 to 34, so the guy who is 5'6" and the guy who is 6'4" both get pants that fit without a trip to the tailor.
Which color for which trade
White is the painter's pair by long tradition, partly because it shows a clean crew and partly because drips of white paint disappear on it. Black and dark gray twill hide grease and suit mechanics and kitchen crews. Brown and olive are for dirt. Stonewash and dark blue denim are for everybody else. The CA8929 comes in five twill solids and four denim washes, which covers all of the above.
Shop the CA8929 in all 9 colors
And if you never swing a hammer in your life, the loop will still hold your keys. See the rest of the Oscar Jeans line or go straight to Work Utility.