Obviously a spare wheel and tire weighs xxxlbs, but UNSPRUNG weight is different.
Think of it this way, SPRUNG weight is everything the vehicle springs are holding up. So, the body, the engine, the interior, You etc etc....
UN-SPRUNG is everything Not sprung, ie, below the springs and above the road. Thing like wheels, tires, axles, brakes etc..... All those items are Un-sprung.
Now, obviously that weight has a lot of momentum when bouncing around all over the road. That why we need Shock Absorbers.
The heavier the un-sprung mass is, the more momentum it has, and therefore the heavier the shock absorber valving needs to be to control it all.
That's why high-end vehicles have super light weight Forged alloys that are lighter for their strength, its in an effort to reduce Unsprung weight, allowing a softer damping (shock absorber) valve setup for a better ride/handling compromise.
With items that offer rotational mass, such as wheel and tires, it of similar benefit that those items are lighter too. Becuase obviously, every time you accelerate or brake, that weight needs to be slowed or accelerated, using fuel or braking power.
every bit of weight has a flow-on effect, and for argument's sake, 40lbs of spare tire added as SPRUNG weight will have a LOT less effect than 40lbs of UNSPRUNG weight.
Sorry for the disjointed explanation, I'm in a bit of a hurry, supposed to be working