I agree with her. In our family, it's not the week-long child-care arrangements that have to be set in place - that gets sorted out before mum goes back from maternity leave. It's the ad-hoc and after-school arrangements that depend on me being around to help out. If a child is off school when parents have to work, mum is working late, children need picking up from after-school club, half-term, it's grandparents that cover those times. There's be no point going to work if all of these additional times had to be paid for, too.
Most importantly, the children have a close attachment to me and when they are tired or unwell, the last thing they need is to be with an after-school assistant or holiday club supervisor, no matter how lovely they are. If they can't cuddle up to mum, then grandmother will do very nicely. I would have been unavailable for all that if I had carried on working.
Working out the time spent on unpaid child-care over the course of each month, I reckon I have 4 of my 6 grandchildren in my sole care for about 30 hours a month, not counting the occasional sleepover.