constant temp circuits is the boiler flow temp circuit, typically for radiators.
variable temp cct is where a lower temperature than boiler flow is needed for example
boiler set at 80 degreees, constant temp to hot water cylinder, and radiators, (which in turn are self regulating (t.r.v.s and cylinder stat))
underfloor heating on variable temperature maybe 55 (controlled by a 3 way motorized valve that sends boiler flow directly back to the return when the water in the cct has reached the temperature set point) or maybe in a schooll where they only want 40 degrees in the rad.
the boiler is designed to operate at max effiecency at a certain temp, be it 60 or 90
using this vt cct you can obtain a desired temperatue in a designed zone, at 100% boiler effiecency
not very good at explaining stuff, but i hope you get the jist.
shaun