Email before were only available in 24 different domains. This fully enough to create millions of possible email accounts. But to make the box more various, we have added 104 new domains that can be selected by choice or randomly generate. In sum currently available to 128 domain names. 

The more domains are more possibilities for your use especially if you have many accounts on one site.
You can register more users with different Emails.

Also do not forget that fake Email service continues to run even when you are not on the page. so you can remember or invent e-mail address for an existing domain (which can be found on the page in the dropdown list) and take the right post. Mail will be stored on the server and you can read it at your convenience 

1) You can change on generate name of email.

2) You can select domain from dropdown list.