I've seen many and not thought too much about them, so a part from the wheel and tyre size being larger, the speed restrictions being the same, why have a full size wheel and tyre and restrict its speed capability?
My theory: because the tire/rim is usually not a matched size (i.e. for my polo it's 185/60R15 while the rest of the car is 215/40R17).
Having an unmatched tire is not very desirable for driving around a lot/very fast.}
Furthermore the chances of the spare tire being lower in pressure is also real, so all in all a less-than-optimal situation.