What the type Reserve "means" is when you play it, you put it on your Tavern mat.
But this by itself seems as arbitrary as a thing to give a type to as any other thing that cards do when you play them. Cards with "trash this" don't have a type; Cards with "+1 Action" don't have a type; etc.
Knowing that, except for Coppers, only brown cards can be on your Tavern mat is incredibly useful to prevent mess-ups, just like knowing that those orange cards might stay in play for more than one turn.
You can of course argue that the text is all that a card needs but types and colours and icons (and keywords, +1 Coffers is far more elegant than take a Coin token) are shorter than walls of text. Just imagine every Treasure card being as white as Action cards and featuring the text: you can only play this in your Buy phase. That would reduce ease of play quite a bit.
I don't disagree with the idea of the coloring; especially for things like reactions and durations. Not as sure about reserves, since the Tavern mat isn't all that different from the Native Village or Island mats. But really what I meant was that I see "type" being used in different ways for different types.
When it is used for action, treasure, night, victory, and curse, it means one thing, while when it is used for for duration, reserve, and reaction it is a different thing (color to help it stand out), while it means yet another different thing when used by all the other not-often-used types. I just would have liked to seen the "primary types" get something distinct from all the "secondary types". Durations and Reactions could still have a different color under that system.