Shielding

Shielding :umbrella:
(Did a search but didn't find a conclusive answer).

I am on the verge of building an all valve pre (line and phono) amp. The electronics are not the issue they are tried and tested. What I am concerned about is the chassis, especially the shielding aspect of it.

1) What is shielding meant to do exactly?
Does it have to keep things (RF, magnetic fields,…)out or in (or maybe both)?

2) Do you have to connect it to the earth or not?

3) Is it equally important to all parts of the amp? Or is it perhaps even better to leave the power supply out of it?

4) How important is it anyway?

Hope you can give me some answers
 
To Shield or not to Shield

I am just a novice in electronic world, but I built a pre amp based on Marantz 7 design. I found that the R-Core Transformer that I use, employed a copper plate wrapped over the winding. I "Guess" it works as a shielding too. Other than that I do not put any special shielding in the pre amp. In the case of "ground", I also need to connect the ground to the chasssis in order to get rid of the humming. Now, it is completely silent (as far as I can hear). Hope it helps.