Quite a number of drivers, particularly the ones who picked up bad driving habits from other drivers who didn't know any better, often practice improper driving by not shifting to neutral when the car is not moving. While I would like to tell you that there is a good reason (under normal circumstances) for keeping it in D and stepping on the brakes at a stop, I can't.
It's mostly born out of laziness and improper training.
As you have correctly surmised, the engine does consume more fuel and you do wear down your brake-system components more when you leave your transmission in D or Drive while the traffic light is red or when you’re idling and waiting.
With automatic transmissions, the engine is energizing or driving the transmission to move in the direction of the gear you've selected--forward or reverse.
When you keep it in gear, you are telling your vehicle to move;
when you're on the brakes, you're preventing motion. You're unnecessarily and slowly raising your automatic transmission fluid, wearing out your transmission clutches, and consuming more fuel because you're on the brakes to keep the car from inching forward. You're also unnecessarily wearing down and heating up your brake pads. By simply shifting to N or neutral there would be no need to apply the brakes with the amount of force required to resist forward motion.
Should you really shift to neutral when stuck in traffic? - Motormouth Online | TopGear.com.ph