Like you, I don't really see the point. My kids solid poo has always rolled off (sorry TMI). I have only ever used them with terrible terrible nappy rash that needed a zinc based cream. Or in fitteds where the bamboo is straight onto the skin (so the liner acts to draw the moisture away from the skin). I hate the flushable ones. They feel revolting to me, so I'd hate them on my bum. I've only ever used micro fleece.
TBH I think the reason many people use them is to try and prevent staining so they retain some resale value.