Not all light fittings and lamps (bulbs) are designed to operate at mains voltage which is 240V in the UK. One of the most common types that have been around for many years are low voltage fittings which operate at 12v, such as ceiling downlights using MR16 or MR11 lamps. These require transformers to reduce the voltage from 240V to 12V.
All LED light sources cannot operate at mains voltage and need transformers (also known as drivers) to transform the voltage down to a level that they can cope with. You may think that not all LEDs have drivers as some LED replacement lamps, such as LED replacement GU10s, have a driver that it built-in to their twist and lock caps – they are still there but are hidden. Some LED fittings have integrated drivers but many need remotely located drivers.
These need forward planning to ensure that they are close enough to prevent voltage drop and are in a serviceable location in case they develop a fault – who wants a section of their ceiling removed or tiles taken off a wall to access a faulty driver? As a Lighting Consultant I usually liaise with my client electrical contractors to work out suitable driver locations although some are happier to make these decisions on their own.