Great question — answering as the UphillTech team:
It was never required, but done for keeping count of passengers. Electronic boarding passes made counting paper boarding passes irrelevant. The following is a story from a user on Quora : This is coming from the experience of working at the airport. As you know passengers are given a boarding pass at the check in counter. By the boarding gate an agent scans the boarding pass, then tears the boarding pass and keeps the bigger part (we used to call that coupons). And while the flight is boarding one staff member would separate the coupons into economy, business and first class while counting how many coupons in each class. Every few minutes the staff member counting the coupons would tell...
Full answer by slingeraap: https://travel.stackexchange.com/a/203753 (CC BY-SA 4.0).
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