A recessed switch or socket presents an easy path for fire and smoke to spread from room to room.The Electrical Safety Council Best Practice Guide recommends fire stopping of socket box openings. Building Regulations Approved Document B states that any penetrated fire rated wall should be reinstated to its original fire rating.
Reduce Noise Transmission.
Wall penetrations allow sound to travel, reducing acoustic performance - which should be reinstated as per Building Regulations Approved Document E.
Improve Energy Efficiency.
Wall penetrations allow heat loss and un-insulated recessed wall boxes offer a path for air leakage. When there are a high number of units in each building this can amount to a high total air leakage. Many building are now specifically tested to meet air leakage criteria before being "signed off" - this is covered in Building Regulations Approved Document L.