jdom84 wrote:I mean I thought it should not have any hum if its a decent grounded set up surely..
The hum will mainly come from the guitar pickups. Even humbuckers will pick up some hum. Because the two coils are physically displaced, even though it's by just a very small distance, they'll pick up the noise waves at very slightly different phase angles, so hum cancellation isn't 100%. Active pickups are quieter, but even then, its hard for them to not pick up any hum at all.
On a clean guitar signal, the hum may be so low in level as to be unnoticeable. But it's because a distortion pedal can apply so much gain, that the hum still needs to be taken into account. And just because your guitar may be almost hum-free doesn't mean that someone else's is as quiet, so the pedal designer needs to assume a fairly bad worse case.
In the above crude sketch, fig 1. is supposed to be a guitar signal with a low level of hum .
In fig 2, a reasonable amount of gain has been applied, making the both the guitar signal and the hum signal much larger in amplitude. The hum and the signal level are still in the same ratio as before though.
In fig.3, the amplified signal has been passed through the diode clipping stage. This cuts the signal peaks off and limits them to the diodes forward voltage value (about ±0.7v for silicone diodes). So although the signal peaks were a lot greater than the hum, now they've been clipped and limited to a value that's only just greater than the hum level. Now the hum is very noticeable and it will interfere with the guitar signal.
That's all very simplified, and it may well be that in some instances that you'd still not notice the level of hum, but in many situations, you most definitely would. So that's where the high pass filter comes in, reducing the hum level (but also the overall amount of low-end in the guitar signal).
This reduced low end is generally seen as beneficial, so is accepted by most, but there are times (as when you play solo) when more low-end would be welcome to the player.