Tri-atheletes and time trialists use them quite differently.
The tri position (as used by the run/swim specialists) has less bend in the waists, but is rotated forward. aerobars are used to rest shoulders after the swim, and the bikes should have more stable steering to compensate.
TTers simply get very low, and bend their waists more. There are UCI restrictions on how far you can rotate your posion about the bottom bracket.
They are usefiul for turning an MTB into a touring bike, you dont have to adopt a low position to take advantage of aerobars.
They also work better with bar-end shifters than ergo-levers.