Thats what it's all about! My last birth started at 4pm and I didn't get home until 11pm the next day. So if my brain is functioning, that's a 31 hour birth in which you don't sleep or leave the birthing mother, except for food / toilet breaks when appropriate. Who's going to offer you that sort of care? Not all births are that long, but we never know what baby has in store, so the simple fact of knowing she's going to be there through thick and thin is a great reasurrance for most women I work with.