Currently when creating a mail alias, it only lets you forward it to your main mail. If you were able to select one of the extra email addresses you created on your account, you could automate Filters based on the sub-mailaddress it gets in to, and thus automate the handling of that email accordingly (placing it in a certain folder, add a label, etc...)

Example: Currently when I add an email alias, in my Filters I need to add it to a list of all kinds of aliases for that specific filter. If instead I could forward the email alias to a sub-mailaddress, the filter would only have to listen to that specific mail address to work. Moreover, whenever a new alias is created that forwards to that specific sub-mailaddress, the Filter will automatically be in use for that new Alias.