Rootfs is a special instance of ramfs, which is always present in Linux 2.6 systems. It's used as a placeholder inside the Linux kernel, as the place to start and stop searching the doubly-linked list of ?mount_points.
Most systems just mount another filesystem over it and ignore it. The amount of space an empty instance of ramfs takes up is tiny.
See:
CategorySystemAdministration | CategoryRedundant: merge with RamDisk