The purpose of the signature is that you sign it when you get the card, then if it gets stolen, they check that signature with the signature of the person signing it.
No, it signifies your acceptance of the card terms. Nobody is qualified to make an accurate comparison.
I picked up my grandmother's medicine from a costco pharmacy for a while. One time, there was some question of whether she had picked up her medicine or a charge or something. To resolve this, the pharmacist printed a sheet of the latest signatures associated with her account. All were indeed mine.
No one was ever checking. It would have been pointless to try.