Spark Plug Metal Washers

Hi, I'd like to replace the spark plugs on my 198 slant six. I can get ACDelco spark plugs no problem at my local parts store. I've read on a few posts that the metal washer at the end of the spark plug's threads should be taken off.

Three questions:

How do I remove the metal washer from the plug without harming the threads?

What difference is there when the washer is out v.s. in?

Finally, If I replace the plugs, should I also change the spark plug wires as well?

Thank you in advance!