I found an empty bottle, the description is "there are many unusual items in the game, this is one of them." Does anyone know what this is for? Does it have a purpose?

They can be used to make potions in SoU if I remember right, and probably also in the original OC. But they don't serve any essential purpose.

Tal, I thought that was a feature added by HotU.

It was in the OC, at least partly. In Quint's tower in Charwood there is an alchemist's table with an empty bottle and instructions on making clerical potions.

Rastor, I meant via the alchemist tables and various similar apparatuses, not by the character himself.