A mains’ spike/surge can look something like this :

So what’s the big deal ?

The problem is that the spike can be of any size. The taller it is, the more voltage is supplied. Generally, power supplies in a PC have limited protection circuitry to deal with some of these spikes, however spikes can get through and this is clearly not healthy for the individual components.  The majority of the time, one or more components will fail immediately and although this will crash the computer, it is a definite fault that can be dealt with.