As every "evolution guy" says, "Our tailbone is a remnant of our evolution"
But a site says, "It helps you about sitting"
Does our tailbone really help us sit?
According to an online article [See Reference], “The tailbone derived its name because some people believe it is a ‘leftover’ part from human evolution, though the notion that the tailbone serves no purpose is wrong,...” A look at an anatomy reference book will reveal that many muscles, tendons and ligaments attach to the tailbone (a.k.a. coccyx); these appendages serve to stabilize us when we sit.
An online visual can be found here.
And if there is still any lingering doubt about the tailbone helping us sit, sitting is certainly cumbersome when the tailbone is unhealthy (coccydynia).