It's called the "Carb Heater Circuit" and is designed to create a bit of heating by channeling hot coolant around the outside of the carbs to try to prevent the effects of carb icing in winter (or summer in the UK
).
My bike suffered from it when i first got it, but fortunately when i phoned my mechanic when i was stranded by the roadside he knew the symptoms that i explained and he told me what it was.
You're better off Googling or YouTubing it for a better explanation than i could give, but with me i'd leave home for work and about a mile up the road in exactly the same spot my bike would suddenly run very rough, and i'd have to pull over and keep the engine running by revving it as best as i could.
The heat of the engine would then melt the ice in the carbs and i could then be on my way, but that could be a good 10-20mins later so was a tad annoying, and embarrassing.
It was a more common occurrence when there was moisture in the air and the temperature was roughly between 0 to 5 degrees.
I tried buying the recommended treatment, which was a cap or two of Silkolene Pro FST but that made no difference whatsoever.
Then after a bit of research i found that a lot of people used 99% Isopropyl Alcohol in their fuel tank as a way of combating the problem, at around 1% of the fuel tank size, so a 20 litre tank would need 200ml.
Not only does the alcohol lower the freezing point, but will also bind with any water/moisture in your fuel tank and burn it off as fuel.
I still use it every 2 or 3 tanks of petrol during winter just to keep the tank moisture free, and it makes no difference to the performance of the bike plus does no harm to any pipes, etc, as it's so diluted.
Those carb heater pipes are very thin though and the channels have a reputation for getting clogged up (so my mechanic tells me) so it's another good reason to try and have a well kept coolant system.
Doing a proper clean out using Radflush every 12 to 18 months will certainly be a major benefit to the engine and prevent clogged waterways which in turn causes overheating and then more serious problems.
A good working airtight coolant system shouldn't even need to be touched, and the coolant level in the expansion tank should be the same more or less every time once the bike has cooled down, although you may lose a tiny bit in very hot conditions.
NOTE: Carb'ed propellor planes suffer from carb icing too, unfortunately they don't get the comfort of standing by a supermarket waiting for the ice to melt in their engine, but do have their own way of heating up the carbs.