Think a pause between the heat source and the circuits.
A buffer cylinder mixes all the water coming back from active circuits. It also forms a bit of a battery for thermal mass.
Buffer cylinders keep a constant, more even temperature travelling through the heat source and active circuits. A larger thermal mass in the cylinder means the heat source doesn’t have to work overtime to compensate for any temperature variations brought on by circuits being closed or opened. This slight pause on the luxury warmth merry-go-round keeps energy input down at its lowest and most efficient.
In particularly cold climates the buffer cylinder also provides de-icing, start-up energy and guarantee a minimum flow rate throughout the system.