For example, the British famously wore bright red which made them stand out rather than greens or browns which would have provided some camouflage.
Identification and morale, as the dimensions of combat in the days of musketry aren't quite the same as the ones of today. And sometimes, just outright fashion. u/StoryWonker and u/dandan_noodles explain further in this thread. As always, should anyone have more insight to contribute, further posts are always welcome!