It’s a preamplifier so it controls volume and does source selection ahead of a power amplifier. As it’s passive it controls the volume without using powered amplification, it will likely use either an attenuator or some transformers. Generally it doesn’t plug into the wall but if if it does the power is just for displays or lights, it won’t be in the single path.
Positives are that they are supposedly more transparent than powered options. The negatives were always that there was an idea that they were perhaps less dynamic but I’ve seen this challenged with more recent models.