Use of Spiegeleisen in manufacture of steel

I recently read the following statements related to Manufacture of steel via Bessemer Process:

When no more $$\ce{CO}$$ is produced (i.e. blue flame disappears) a calculated amount of Spiegeleisen (an alloy of $$\ce{Fe, Mn, C}$$) is added and distributed throughout the mass by blowing air for some time.

Why do we need to add Spiegeleisen at the end? Is it to reduce the oxides of iron (that might have been formed)? Or due to some other reasons, like, say $$\ce{Mn}$$ to make steel harder and increase its tensile strength? But latter doesn't convince me, since we could have added $$\ce{Mn}$$ directly on order to get the same results. Also what's the meaning of calculated amount, how do we calculate the amount to be added, and why?