I think what OP meant is, if you have a page that's backed by disk, then when you're low on RAM, the OS can simply drop that page -- it's sitting there on disk

I think what OP meant is, if you have a page that's backed by disk, then when you're low on RAM, the OS can simply drop that page -- it's sitting there on disk if you ever need it back. But if you have a page that's not backed by disk, then you have to write it out to disk before you can drop it.