TSA and signing mobile boarding passes [duplicate]

If you go through security at the airport with a paper boarding pass, the TSA will look at your ID and then sign the pass with a pen, but if you use a mobile boarding pass, there is nothing for them to sign (At least as I understand it. Maybe I'm wrong.) Why is the signing necessary in the one case and not the other? Where is the security compensated for in the mobile case? Or what purpose does signing the paper pass serve?