why taking in and out is done at certain seams and not in others

[Fustagno]

I have been watching several videos about taking in (or out) suit jackets, the first place they all seem to work on is the seam right behind the armhole.
Why there is a preference to work on that seam as opposed to the side seam (under the armhole)?
It seems to me that altering just the back would tend to distort the front by pulling it or releasing it. Wouldn't altering the side seam change both the front and the back at the same time thus maintaining more the proportions of each?
(and also because when I lose weight its more a matter of sides and belly rather than back)
Is it because the pocket is close to the side seams?

Thanks

[Schneiderfrei]

The first thing I can think of is that the front presents an image. You want to disturbe that the least.

Next a lot of fitting issues involve the rear scye. So dealing with that first makes good sense.

I hope others have some better ideas.

[TTailor]

The underarm seam goes through a pocket.
If the jacket generally fits, the shoulders, shoulder slope, back scye, and back width are areas that are both more accessible and often the areas where some fitting is beneficial

[Fustagno]

Thank you all for the answers

[Greger]

Tailors sometimes move the back up or down on the side seams for balance adjustments. Inlays at the back armholes are way easier to work with. Gouging out the front armholes is way more complicated. Do as little as possible, because it is very hard to correct if done wrong, or expensive to replace. Plus speed. Tailors are often paid by the job and not the hour. The more coats you finish in a month is more money.