Why do we still have tailbones if we stopped having tails ages ago, when we evolved into apes? It’s really annoying

Question: Why do we still have tailbones if we stopped having tails ages ago, when we evolved into apes? It's really annoying when I lie on my back.
- Keywords:
  body,
  evolution,
  human
Asked by gallifreyanhippo to Aled, Ellie, Fiona, Kev, Willem on 11 Mar 2014.
- Kevin Arbuckle answered on 11 Mar 2014:
  
  Our tailbone (or ‘coccyx’) is much shorter than in species that actually have tails, and the reason we still have it is that some muscles (including your bum) actually use it to anchor to the skeleton. So it still has a function in humans (and other apes), but even a short one is enough to be useful in its current role.

Question: Why do we still have tailbones if we stopped having tails ages ago, when we evolved into apes? It's really annoying when I lie on my back.