The current raiding system has an issue for users that are willing to join a raid but are unaware if others are. Depending on the situation or location you may not always be able to find others that are playing. The only way to show that you are ready to raid is to use your raid pass and for some that may be their only pass for the day. Effectively wasting it if no one is around.
By adding in a check-in feature users can show others that they are willing to use their raid pass if others are too.
Additionally, you can see which team players are on for those that have checked in. This would be beneficial to those that would want the chance of an extra premium ball from having higher team damage.
The Check-In button would be featured next to the Battle button and would only be interact-able if the user is within range and has a raid pass to use.
From the outside, the raid-level icon would alter between the current state and an updated count of those that have checked-in so nearby players can rush over to join in knowing there are already players ready to raid.