system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Trek] Starfleet seems to totally change its uniforms every few years. What justification does it give for this change? Do they just blast out a subspace communique to the fleet saying "we have decided that grey with ridges is the new red"?

Is starfleet really that fashion savvy or are there pragmatic improvements being made with each swap?
gpt
If we were to paint Starfleet uniform philosophy with broad strokes, we can see a back-and-forth battle between uniform uniforms (uniforms that look mostly the same, with minor differences to denote rank and division) and uniforms with drastically differring appearances.

Why? Well we can only speculate as there is no provided blanket "justification." All we see are changing uniforms.

Uniform uniforms are all the same for a reason: you want to create and enforce a "group" psychology: aside from minor differences, we are all part of the same team fighting for the same goal. Key word: fight. Having rank after rank of soldier all look the same is a key element in a unified fighting force.

With United Earth (and later Stafleet), the uniforms were this, and it makes sense. Earth had just recently risen from the ashes of World War III. A combative mindset was still in the forefront and uniform designs reflected this.

The birth of the Federation brought about relative peace and security and Starfleet branded itself an organization whose primary mission was exploration, not combat. To makes appearances coincide with this goal, the uniforms changed to be less militaristic. This brings us to Kirk-era with the division colors being most prominent. Rank insignia was down played, and the breast insignia was tied to the ship, rather than the organization.

But Kirk's era was not all sugar and candy. There were ongoing hostilities with the Klingons, not to mention a brief incursion by the Romulans. Probably not conicidentally, we see a gradual shift back to the Uniform uniforms, this time red being the primary color.

This carries us from the late-Kirk era all the way to Picard era. This, not surprisingly, is a period of increased hostility between the Federation and Klingon empire. Despite the Khitomer accords and "official" peace, the Klingons would still be antagonistic toward the Federation until the events of Narendra III, in which a strong and lasting peace and alliance was formed.

It was during this time period that the Klingons settled their issues with the Romulans (at least on the battlefield) who then retreated into a long period of isolation. So, relative peace again settles on the quadrant.

Not surprisingly we have a uniform change back to solid colors of different departments (with the infamous yellow-red swap).

This pattern holds until, again, war breaks out. This time with the Dominion. Another change back to the Uniform uniform.

So it would seem that, when everything is fine and dandy, the Federation wants to distance itself from the military look of their personnel (while still actually having a uniform), but when the blades come out, they revert back to cookie-cutter uniforms that look mostly the same.