For example, your company has held a Christmas event and created a poll for the participants to vote for the Best Santa. To make sure the vote result is accurate and non-controversial, you must wish to prevent those who don't join the event to cast their vote. Hence, setting the participant type "Event" would serve the purpose.
May refer to the link to learn how to check in an event and check the attendee list:
EasyWork - Event - How to scan for attendance from the company event?