Husbands don't have to cheat and leave for long term SAHMs to be up shit creek either.
My dad didn't, he and my mum had a fantastic marriage, he worked all hours god sent and she looked after us and home. While it lasted it was wonderful. And then he got cancer and died.
I have very vivid memories of my older brother trying to get mum to tell him what she had eaten that day to ensure she had eaten that day when we were sitting down to dinner. I can remember the times when even her going without wasn't enough for us to have enough to eat. I can remember that same older brother desperately trying to keep it all together and be the 'man' when he was still really just a child.
My mum is a tough woman, but she found it damned near impossible to find paid work after being out of the work force for so long with us.
One of the things I learned was that you couldn't rely on somebody else totally - no matter how much they may want to look after you, shit happens.
If DH died tomorrow, we would be OK. Obviously devestated but OK. We would not need to move and we would not go hungry. We think about it like 'hedging our bets' and creating a buffer. Same goes for me.
While the money I make is not necessary for our day to day lives, it could end up being so. It could pay for DH to have a long time off with his sons if their mother died, it could mean that if their dad died their mum hadn't been unemployed for 9 years and had no relevant up to date experience.