Why would thin person need long vertical TT scar?

Good question! In order to get the horizontal scar as low as possible, and because the aperture where the belly-button used to be will become a hole, a vertical scar is sometimes used to address that hole. Depending on how far the skin around the prior belly-button can move or reach, that determines the vertical scar length. People with a lot of laxity don’t need a vertical scar at all because that belly button hole can reach the horizontal scar

The only person who can answer that question is the one who created that scar, your surgeon. If you do not understand his/her/their explanation about why that scar came to be, ask again for the answer in terms you understand. We add short verticals in some patients to keep the horizontal low to accommodate sparing bikini styles. We minimize vertical scar length by extensive use of progressive tension sutures, operating table flexion, discontinuous undermining, and other measures. In agreement with Dr. Edelson, a year or longer after your initial procedure, it may be possible to shorten that scar.

A vertical scar is needed under two principal circumstances: one is that the skin is not only loose vertically but horizontally. So if the tummy tuck only pulls the skin down, the belly skin will still be loose when you move it side to side and may bunch up. This is particularly true after massive weight loss.
The other circumstance is when the patient has a tall torso and the lower incision is far from the belly button. In that case when the skin is pulled down, it will be too tight to close if there is no vertical component where the belly button used to be. I hope you got your answer.