When you post an announcement, students receive notifications about it on their Blackboard home page. If they are using the Alerts and Notifications tool, they will also receive a notification about the announcement in their email. Checking "Send announcement as email" automatically sends a copy of your announcement as an email to all users in the class, even if they have the Alerts and Notifications tool turned off.
Sending an announcement as an email is a good way to ensure that students are receiving your notices, especially if students are unfamiliar with accessing Blackboard just yet.