To stay sane (more or less) and functional (mostly) in the face of Life flinging shit in your face. If you want some good examples of 'dark humour' ask any survivor of a lengthy traumatic experience, such as a war. If you can find any and get them to talk about it, you will be shocked at what a surviving worker in a Nazi Death Camp found funny 'at the time'.

"we need humour like we need feet. To stop our legs from fraying at the ends."

No - 'To stop our mind from fraying at its end.'

As quoted above

"When you see me laughin'
I'm laughin' to keep from cryin'"

That's called hysteria.

Been there, done that, got the T-shirt...