In short, email addresses are a lot easier for your attendees to remember than a custom username. Additionally, it also looks a lot more professional when displayed on a profile (as a businessperson you would probably rather be seen as rather than _JohnDoe123_). Finally, emails are also a unique identifier for each person that lets you know who they are either through simply looking for their name in the address or by sending them an email. A username might not contain their name and cannot be used to send an email or other form of message.