Trench Coats - what are they actually for?

I’ve been eyeing this gorgeous trench coat on The Real Real but every time I want to go for it, I pause. I feel like trench coats are considered such a classic wardrobe staple, but I genuinely don’t understand the use case. If it’s cold, I’m going to wear a thicker sweater or a knit jacket. If it’s raining, I’ll wear my Barbour.

So what situation is a trench coat actually the right choice? Is it meant for warmth, rain, wind? Light layering? Sorry if this is a silly question but I’m trying to understand it so that I can justify the purchase 😂😅