In historical dramas and contemporary paintings, you always see 18th century soldiers wearing what would today be considered the fanciest of dress uniforms with brightly colored coats and ornate hats. Was this really how the average soldier would dress in battle? It seems like equipping thousands of soldiers with these uniforms would be a significant cost without any tactical advantage. It also seems like they would have been very difficult to maintain and keep clean in a field environment.
See, that's today. The key here is not to look at the situation with a modern eye, but instead to consider the conditions prevailing on the battlefield in the 1700s to 1800s. Consider 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, it's 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!