Very early on in the British electrical industry it was decided to standardise and simplify everything as much as possible for uniformity and safety.
The "ceiling rose" is a clever little junction box that we probably take for granted here, but it makes the lighting wiring very straightforward and versatile. Even if it's removed and replaced with a junction box above the ceiling it still results in a standard wiring arrangement to work with.
It's not just a junction box though. It also acts as a decorative and accessible base for a fixed or hanging pendant lampholder and can accommodate quite heavy light fittings like tiffany glass shades. The screwon cover can also be painted to match the room decor if desired.

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