Because not all email addresses are meant for email marketing.
We let you use almost any address when creating your E-goi account, except for a handful which would seriously hamper your deliverability!
These are known as role accounts and are usually assigned to company departments (eg. billing@, help@, webmaster@, etc.). Never use them for marketing purposes, as they don't embody you as an individual person (but rather a role) and may not even process replies from your contacts (which is essential for any of sender you add to E-goi)!
This means you should always go for your own registered email address (such as firstname.lastname@example.org). Alternatively, you can sign up for an E-goi account using a personal freemail (eg. Gmail, Outlook.com, etc.), but try not to use it for your email sends.