[Amps] cathode to ground in tetrode amp

[Partain, Chuck]

OK, getting into this more and more every day and learning. Looking at some designs on this GU-84b
amp, I see a lot if not most of the cathodes have resistance, some a lot(30k) some little (.7) to ground.
The screen grid and control grids are not grounded and have respectively ~350 and -150v on them.

Why wouldn't it just essentially go to ground? what does the R do in series?

thanks as always.

KA1MWP