The plumber
The plumber moves or extends pipework for the new layout, fits the bath, shower, WC and basin, connects waste and supply, and tests for leaks under pressure. On most jobs this is the trade with the biggest knock-on effect on everyone else, if the plumbing isn't finished and tested, the tiler can't seal around fittings and the electrician can't safely wire anything nearby.
When the plumber is managing their own separate business and diary, any delay here delays every trade that follows.
The electrician
Bathroom electrics need to meet Part P of the Building Regulations because of the water risk, covering lighting, the extractor fan, shaver sockets and any underfloor heating. This isn't work that can be done casually, it needs to be certified.
When the electrician is a separate subcontractor booked by a general builder, they're often juggling several jobs and slot your bathroom in around other work, which is one of the most common causes of a fitted bathroom sitting half-wired for a week.
The tiler
Tiling can only start once waterproofing and boarding are done properly, and it needs curing time that can't be rushed. A tiler working on a fixed day rate has every incentive to move fast; a tiler who's part of the team responsible for the finished result has every incentive to get it right first time.
Large-format tiles, mosaic borders and wet-room floors with falls to a drain all add time and skill requirements here.
The plasterer
Not every bathroom needs a plasterer, boarded and tiled walls often don't, but ceilings, exposed brick after removing old tiling, or rooms being replastered around a new layout do. Where needed, this work has to happen before final electrics and tiling, so it has to be scheduled correctly in the sequence, not squeezed in wherever a plasterer happens to be free.
Why the blame game happens with separate trades
When four independent tradespeople are booked separately, usually by the homeowner directly or loosely coordinated by one lead trade, delays and mistakes tend to get blamed on whoever isn't in the room. A leak behind a tiled wall becomes 'the plumber's fault' to the tiler and 'the tiler's fault' to the plumber, and the homeowner is stuck in the middle chasing both.
With one in-house team responsible for plumbing, electrics and tiling, that argument doesn't happen, because it's the same company accountable for the whole result, backed by a written guarantee.