An email alias is an additional address that routes mail to a real mailbox or destination. Aliases are useful when you want multiple public-facing addresses without creating a separate full mailbox for each one.
For example, you might have sales@yourdomain.com and support@yourdomain.com forward into the same mailbox, or route them to different users depending on your workflow.
Aliases are helpful for organization and branding, but they are not a substitute for a full mailbox when a user needs separate login access and stored mail.