There are a myriad of reasons you might want to do this. Maybe someone thought it was a good idea to commit binaries and now your repo is enormous and takes 30 minutes to clone. Maybe someone accidentally or otherwise commited a (but likely many) passwords, and now even though you've removed them, they are still retrievable through the commit history.
Either way it's now up to you to do the one thing git was designed not to do: Forget.
You will be making changes to the history of the repo. This will cause problems for others working on the repo in parallel to you.
- Make a plan for a code freeze.