Picture a host by each door greeting people and taking attendance on an iPad or their phone as people walk in. Or maybe someone is in the sound booth, looking over the audience, checking in on the people they see and know.
Another scenario might be an event for which name tags are pre-printed. As people pick them up, a host clicks the box by their name to check them in.
The benefit of check-in by roster is that there is no need to search for people, you simply scroll to their name to check them in. This is a much faster and less involved process.
This hidden gem from last week's new UI release can be found by creating a check-in view for the service or event (more info). Then start check-in using that view and click the reports option in the bottom right corner. On the next page, click the Detail Attendance Report. After that select the new Full Roster option (circled) and you'll see the check-in by roster.
Leveraging the detail report like this allows you to do check-in at the same time on multiple devices. They share information without overwriting one another. If someone is checked more than once, the system recognizes that and won't duplicate attendance.