A description of ECT poorly applied in the mid-1950s, from The Bell Jar, pg. 143: [i]
I tried to smile, but my skin had gone stiff, like parchment.
Doctor Gordon was fitting two metal plates on either side of my head. He buckled them into place with a strap that dented my forehead, and gave me a wire to bite.
I shut my eyes.
There was a brief silence, like an indrawn breath.
Then something bent down and took hold of me and shook me like the end of the world. Whee-ee-ee-ee-ee, it shrilled, through an air crackling with blue light, and with each flash a great jolt drubbed me till I thought my bones would break and the sap fly out of me like a split plant.
I wondered what terrible thing it was that I had done.
A note to clients: the use of ECT has really improved since then.
[i] Harper Perennial Modern Classics edition, ASIN B004N8X6LK
PsychotherapySphere 
But why is it necessary in today’s modern age? With psychotherapy, psychotropics, somatic therapy, EMDR, isn’t this saying that maybe they just weren’t working with the right person?
Per wikipedia, it’s used today in cases of severe depression that has not responded to other treatments. When therapy and meds aren’t helping and a client is at considerable risk of suicide, ECT can be the “least bad” choice. Plath refers to spending some time in a hospital receiving talk therapy before her ECT treatments began. It would seem that even in 1950’s, psychiatrists didn’t believe it was necessary unless and until less-aggressive treatments failed.