The reduction is extremely important! So when someone else turns the water on it will have enough pressure to feed both.
Notice toilet flush reduces water pressure in the shower. If it were 1/2" all the way to every access, and you turned on multiple valves at multiple elevations, you'd lose pressure and run the risk of creating a syphon. That would suck air in through the open valve.
Imagine taking a shower up stairs, dish washer going, Wife starts to water the garden, hot water & air start spraying out, shower head becomes a vacuum. Also do the math limited diameter pipe. Can only send a limited a limited amount of water. It's all about supply & demand. It basically holds pressure for when you need it.
If it was all the same size diameter, when you turned on several things at once, the city or "your supplier" would need to turn up water pressure feeding your structure (somehow 🤷) every time you turned on everything.