My understanding is that the number of such sender aliases is limited, at most 50 or 250, depending on the plan. There are ways to use a custom domain for sending where you end up using a larger number of localparts fairly quickly, and it would be a hassle to have to manage them, instead of just typing whatever sender you want (or on replies, having the email client automatically use the address from the original email, without having to worry whether it’s still in the set of registered aliases).

When you have a custom domain you can list @mydomain.com as sending domain allowing you every string before the at character. So that means you could use 50 different domains with infinite adresses on these domains.

The limit is only enforced in the web interface. You can send from any alias using any third party email client, and on the website you can configure a catchall mailbox and create a rule to filter out the aliases that receive spam.