Same reason Europeans wore bright colors until camouflage became a necessity. So it is easy to distinguish friend from foe and to demonstrate to the enemy you're the real thing and not going to flee first.

Remember each soldier has their own lives, perhaps a wife, kids and aged mother depending on their income, even if they're not a coward they will be considering fleeing, or they may well be purely self-interested or terrified. They show up on the field of battle hoping that things go their way, that they can scare the enemy off with a show of force and get paid well without a bullet or bayonet in their stomach.

However if they are suddenly faced with professional soldiers rather than a rag tag band of levies like themselves, it changes the game. And these people are usually illiterate commoners or just ill informed, they need to see it, they need a visual display. Seeing columns of disciplined soldiers in the same uniform, or these men in wild colorful tiger outfits who seem absolutely insane, insane enough to risk their lives just to get to you, it has an actual effect on their minds.

To modern eyes it seems silly, but imagine if you are armed only with a revolver and a bunch of MS-13 thugs come covered in tattoos, you know you're fucked. MS-13 look equally stupid, but stupid crazy people can still be dangerous.