You have an email, let's say john.smith@example.com. It's a nice email address, but it just doesn't fit your personality.
Now you can add an alias here and have john (at loves.money) point to john.smith@example.com. You can hand out your shiny email address john (at loves.money) to everyone, they will send emails to that new address, and you will still receive them at john.smith@example.com.
The same applies to domains. john.loves.money just sounds better than johns-site.net, wouldn't you agree?