Mirrors can increase visual space of a room. In fact, they can also change the energy distribution of a house/apartment through their reflection functions.
They can amplify energy
From Feng Shui aspect, a room should be in rectangle form. If the longth-width ratio of a room is not proper, we can change the visual space of the room with mirror. When the visual space is changed, the original negative effects to Feng Shui caused by the imbalanced length-width ratio is also changed with the change of visual space. For example, if a room is very narrow, we can put a mirror on one of the long walls to make the room look wider. This mirror at the same time increases the energy field of shorter walls, when it increases the visual width of the room.
They can transfer energy
"Blue dragon on the left, white tiger on the right, red birs at front, tortoise in the back" is a basic Feng Shui principle, therefore we should never sit with our backs againt a door. From Feng Shui aspect, doors are Yin. Mirrors are also Yin. Just like doors, mirrors shouldn't be put behind a chair, sofa and other tings used for sitting. Walls are Yang, but if you put a mirror on a wall, this wall becomes Yin. So you shouldn't sit with your back againt the wall.
We know also that "door against door" is a kind of layout which does also harm to Feng Shui. If a mirror is higher than 1/2 of the normal length of a door, the effects of the mirror to Feng Shui are just same as those of a door. Therefore, it is not recommanded to put such a mirror right against a door.