Your signature is not needed. Only your credit/debit card is
needed.
Sometimes your card is not needed. Only your card number is
needed.
Once the card is swiped through the reader, your account is
billed with the transactioned automatically. The transaction can be
billed to your account manually, if
your card number is given in writing.
The onus is on you to prove you did not do the transaction,
therefore the signature works against you. Anyway even if you did
not sign, i.e. you gave your card number, you are by
contract liable for transaction done on the card account.
Once you have a credit/debit card, you have a hole to your money
that makes it easier for the banks to take your money to pay the
merchants. You provide the security by carrying the hole
guardedly.