Four things:

1. It matches the strength curve.
2. It prevents a common technique flaw.
3. It provides a stable base to work from.
4. It keeps the elbows healthy by lining the force up between your shoulders and elbows.

If we look at a standard straight-bar tricep pressdown, one of the major mistakes people make is at the end range. They have to shift their shoulder position to finish the lift. This shoulder roll takes tension away from the triceps.

Secondly, as fatigue increases, this becomes more of an issue, partly because the triceps are tired and partly because the lower traps can no longer control the shoulder blades.

By the end of most sets, people are shrugging up, hunching over, and using momentum to move the weight, all of which does zero for tricep development. But it does beat up your elbows.

To improve these issues, lifters often use a rope attachment which means the hands aren't fixed. This means the hands can move apart more naturally at the end of the range. This will help to keep the elbows healthier, but the same issues with stability and shoulder dominance show up.

To fix these issues you need an exercise which provides a position where you can stabilize the shoulder blades and promote trap activation. Then you want to align the force through the shoulder, elbow, and wrist so that no excessive strain is put on any of these joints. Doing so means the load is driven through the triceps where we want it.

This exercise does all of these things. It does the job of the lower traps so their strength endurance doesn't become a limiting factor. The bands anchor your shoulders in position and basically provide you with an extra set of rubber-band lower traps.

It also centers the resistance through your shoulder. This means the set-up is specific to your exact body structure. You're not fixed into a position that's too narrow or too wide because of equipment limitations. This is a common issue most people will struggle with when using standard cable attachments.

Extension patterns have an ascending strength curve where you get stronger throughout the range. Using bands is perfect for this as their tension increases the further you go.

The solid position, artificial shoulder stability provided, and line of force makes this an awesome tricep-building exercise. It's also a great one to take to failure because the risk of injury is so low.