If memory serves me from my CBR training WAY back when - the significance is two-fold. (1) Smaller quantity of fuel is required, and (2) Thermo-nuclear explosion (what used to be referred to as H-bomb) results from plutonium. Much more powerful explosion - megaton range, as opposed to kiloton range of uranium explosion.
This is only true if the device has a fusion stage (for example, lithium-6 deuteride, in which fusion would occur after being "ignited" by the plutonium fission device; there are other ways to do it as well, but they all involve fusion of light isotopes like deuterium or tritium rather than fission of heavy elements like uranium or plutonium).