Nothing
It increases your awareness, and shows what works for the car and what is costing you extra fuel - that is what makes you save fuel.
The better you work with it, the more fuel you'll save.
That alone will reduce your fuel consumption.
Twofold
- by driving slower
- by braking and accelerating less
No-one's really going much faster in a traffic jam.
These guys want to stop even less than you do, so they're trying to keep rolling, and you can benefit from that.
Do you stay in 2nd gear ?
Try to avoid 1st unless you're actually stopped, and shift into 2nd right away.
If you can't stay in 2nd (or 3rd) all the time, do a short burst in 2nd (pulse) (or 3rd, depending on speed) , then shift out of gear into neutral, and let it roll (glide) . Repeat when speed drops too much / the gap widens too much.
My mileage used to drop like a stone in traffic jams - now using the above technique, it's sometimes going up due to the lower speed
You could use that technique in town as well.
On short stretches of road, accelerate to a speed that'll allow you to roll the remaining distance to the next light / stop-sign / corner, then shift into neutral and coast.
On longer roads, maintain a speed up to a point where it'll allow you to roll the remaining distance to the next light / stop-sign / corner, then shift into neutral and coast.
Hard-core hypermilers even shut down the engine while they coast