Why is the at symbol used in email?

If they are doing it online, it is because there are computer programs capable of adding email addresses to huge spam mail lists, which can be protected against by writing the email address in an unusual way, such as putting the Word at in brackets, or just typing it out, or some other method of making the email address different enough from a normal one that a computer program would not be able to recognize it and start sending spam mail to the person's email address.