If you asked this question from someone back in the day, you would probably get the answer like “to save men from dirt and rain.” Ask it now, and you’ll get several replies. Some love to wear it for “swag.” They usually buy the fancy ones.
However, most people just need something to get through a rough day, so they go for something equally light and water-resistant.
And then comes trench coats made of insulating fabric to give you that extra warmth you will need in chilling winters. Means there’s a huge variety now. And it really depends upon what you require of it.