Traditionally manuscript signature performed the function of either authenticating something or accepting something in simple terms. An example of authentication is signing a cheque to authorize payment to a payee. An example of accepting is signifying that one has read, understood, and accept the content as written by appending their manual signature. When one signs manually in isolation, the only way of personal authentication is to compare the unique way the person usually used to sign and satisfy ourselves that she is whom she claims she is. However, that is not so foolproof to guarantee as no one can sign exactly in same precision many times much less even to a minimum of two times.
Similarly, with the naked human eye, one can hardly compare two manual signatures and confirm that they are the same with a guarantee. We lived with this approximation ever during the era of manuscript signatures but believed that to be the best way of authenticating or accepting. Every relevant legislation leaned on this manuscript signature, and thus life went on.