Why use an Email Alias?
The good side of using email aliases pertaining to a certain email address is that you can set as many of them as you like. Most email service providers do not set limits on the number of email aliases created per email account, which gives users the possibility to create various alternative names for their original mailbox.
For instance, with the help of email aliases a sales email address of a company (e.g. firstname.lastname@example.org) can be assigned individually to all the members of the sales team (e.g. email@example.com, firstname.lastname@example.org). This way, each of them will be able to reply to incoming email messages from their personal address, while those messages are addressed to email@example.com.
While being of great help in setting up a network of fictive email addresses serving one major mailbox, the email aliases do not boast much functionalities apart from that. The reason for that is that with the set-up of an email alias, no resource-consuming mailbox is actually created.