Due to the fact that domain names are cheap, it is recommended to buy more TLD(Top Level Domain) types like .org, .net or .info alongside .com. The additional domain names could be redirected as aliases to show the content or the website designated by the primary domain name. When you get popular over the Internet through a domain name or a trademark reflected in a domain name, the possibility appears that other people take advantage of your popularity benefits too by cyber squatting.
For instance, they can register a domain name similar to yours with the only difference in the TLD type. Then, they could sell it to you with an enormous high price. Or the cyber squatters could gain certain traffic for their websites based on a similar domain name with yours.
If you have a primary domain like "mydomain.com" and you want to define "mydomain.net" as an alias to it, "mydomain.net" domain will have the same properties as "mydomain.com" regarding the DNS and mail. As a result, the email sent to the accounts defined on "mydomain.net" will be managed as if it was sent to a "mydomain.com" mail account type. You can have the advantage of the domain alias by managing the emails from two separate domains mail accounts in the same inbox.