Shielding 
(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

(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