Hiding emails is useful if you want to hide emails from the preview screen to emphasize other emails.
For example, you can hide all emails from those on your friends list, so only spam and undetermined emails are displayed. This makes it quicker to go through your spam emails. Alternatively, hide emails in the email grid from those addresses on the blacklist, filters, origin of spam or First Alert, so these emails are not shown so as to make it easier to read your legitimate email.
An envelope icon is displayed at the bottom of the preview pane showing how many emails are hidden from the mail grid. This icon can be toggled to show or hide the hidden emails.
Spam tools with the ability to hide emails are:
Friends list, under options
Blacklist, under options
Filters, under the Action tab
Origin of Spam, under options.
First Alert!, under options
Each spam tool has the option to only hide the email if the email is not marked for delete, therefore if the email is marked for delete, it will be shown.