A valve (tube) is a capacitive load. Anode (plate) and grid are two electrodes insulated by a dielectric (vacum) so they form a capacitor "Cgk". This capacitor is multiplied with the gain of the vave ( I don'd know the mechanism that causes this, I can tell you only that this is called "Miller effect"). If we tried to drive the input of an amplifier valve with the hight resistance output of a passive guitar (maybe 50kOhm) this would form a lowpass filter, that atenuates the hight frequencies of our guitar and makes the entire amplifier have a poor trebble response.
So if we use a kathode follower, wich has a voltage gain about 1, the input seems a far smaller capacitor (multiplied by 1 and not 60), giveing a much better frequency responce.
Kathode followers also tend to have much smaller output resistance (kathode resistance of a valve equals to the anode resistance divided dy the gain of the valve plus one Rk=Ra/(mu+1) (mu is the aplification factor of a valve). This way they are very powerfull for driving the input capacitance of a voltage amplifier valve creating a very good HF responce of the stages.
Now what about the strange bias?......
A valve's grid is biased with a voltage negative with respect to the kathode, and -5V is a very possible value. For a common kathode amplifying valve, we can create this voltage at the cathode of the valve using a series resistor and then just connect the grid at the groud poetential with a large valued grid leak resistor.
Now for a kathode follower that has its kathode at a hight poetential, about 150V, we must bias the grid at 145V. Such a voltage can be made using a poetential divider, a leg of two series resistors from the HT suply to ground poetential. The appropriate analogy of resistor values gives the apropriate voltage at the node between the two resistors, and finaly, we connect the grid there and the valve is biased.
I hope you understand some of what I wrote..... I know, my english are terrible..........
I advice you to by a good book of analog electronics. This would make such questions a nightmare of your past.... Cheers!