Your location might be useful. I think the UK/Europe do not mix hot and cold water Heated water must be in a closed system or dump down a drain, to not enter the cold water lines.

The Netherlands. We use mixer taps here, I think I have never seen UK-style separate taps anywhere in continental Europe.

It could also be for safety, they do not trust the water in the gas boiler(not treated) or energy savings. It is easier/cheaper to heat up 40C return water than 5/10C replacement water.

@Tetsujin There was another question a few months ago that mentioned the hot and cold water pipes went all the way to the end of the faucet so the two would not mix. Something about the hot water being allowed to mix with the cold water. It was strange to me, since we always mix the two, even in the hot water tank.

I've only just realised that "central gas boiler in the building" is probably in some kind of apartment block, which makes all my reasoning invalid [or at least skewed], as I was thinking 'house'.

Would a central heating boiler be kept running in the middle of summer when nobody wants heating?

@IgbyLargeman the boiler, if isolated well, acts as a buffer. People need warm water even in the summer. So the heater runs not the entire time, but only in intervals to keep the water warm enough for all the people to have their warm shower. That's the reason why dimensioning the boiler is key. It could be optimized with adding other heat sources, like solar.

@crip659 they meet in the tank, because that's fed by cold (either mains or gravity). They meet in mixer taps and shower controls (where they are combined into one hose to the head. It's about how you ensure that mains can't be backfed in the event of a fault. But radiator water is filthy; you couldn't route that to your taps. Instead (in a typical UK house) a primary loop feeds the radiators and a heat exchanger in a tank full of clean water to be heated, with valve(s) to manage demand between the hot water tank and the radiators

The other thing is to minimise the amount of warm water lying around in pipes, in which nasties like legionella can grow

Potable and non-potable water. Or an electrician adding a PoU heater was cheaper than the plumber doing pipe the whole way. Or they found a HW tank in the garbage and put that instead of $1k in heat exchangers (which would let you do P and non-P from the same single system).

@MonkeyZeus combined systems use a heat exchanger to avoid that problem.

@IgbyLargeman I suspect that's the reason -- a boiler with the capacity to heat a house would be very inefficient in the summer if all it was doing was providing domestic hot water.

@ChrisH but with the heat exchanger you mentioned, does it really matter if there's legionella in the closed-loop part of the system?

@LShaver only if you're inhaling over the feed/expansion tank; anyway the closed loop should run hot enough to kill it. But my last comment referred more to pipes feeding showers etc. - long runs full of warm water that might never have exceeded 60°C depending on how fast it flowed through the heat exchanger. They should be flushed (even rarely-used cold taps get flushed here in work)

Not a real answer, but in some older buildings, the boiler/radiator system was installed years before the hot water taps (or vice versa). Bolting on a second independent system is often cheaper and easier than trying to integrate it into what you already have.

@IgbyLargeman It happens to be summer right now and the central heating boiler happens to be shut down, so that's definitely part of the answer!