MailChimp described segments as:
"A segment is a section of your list that includes only those subscribers who share specific common field information.
The beauty of segments is that you’re able to target different types of subscribers in a single list, so you don’t have to worry about overlapping subscribers across multiple lists. Segments can streamline your list management and reduce your monthly billing rates."
Sheep can automatically creates segments for each of your Events. The attendees of the event will be automatically pushed to MailChimp.