Most AC electric motors (induction motors) run at a speed which is a function of the supply frequency and the load on the motor. So as the mains frequency varies, the speed of every induction motor connected to the system varies with it.
At the most basic level, people don't necessarily want their motors to be changing speed all the time, regardless of their load - and lots of load types have a relationship between speed and power which is not linear - fans are a good example of this, where the torque is proportional to the speed, so the power is proportional to the square of the speed. This means that small changes in frequency lead to larger changes in power. It's useful to the network operator that if the frequency drops then the load falls too (falling frequency is a sign that the generators are not keeping up with the load), but you can clearly have too much of a good thing, and it's easy to see how you can end up with everything hunting, as lower frequency caused loads to fall, causing frequency to rise and increasing load, causing frequency to fall, etc., ad infinitum.
Clearly it's easier to set things up to be stable if you only try to do so over a small range of speeds - hence the restrictions.
Besides which, frequency is the one signal of a grid's condition which is immediately available to every generator and every consumer - so consumers can automatically trip-off if the frequency falls too far, in an attempt to help the grid recover from a loss of generation. Generators trip-off if the frequency falls too far, because who wants to be the last person trying to supply a failing power system...