There are more details in the main VeriFLY thread. But just a few months ago a Brit entering Canada from LHR would need about 10 minutes of checks to enter Canada. Canadians about 5 minutes. A family group particularly with younger unvaccinated children could take 20 to 30 minutes to process - remember there was a requirement to provide a quarantine plan in case the traveller turned positive after arrival, a check unique to Canada, along with a non-discretionary rule previously. Many people misunderstood the requirements and that led to arguments, and quite frequently in a landscape of changing requirements agents also would get things wrong. That is unsustainable even with flights only 25% full. VeriFLY allowed checks to take place in your own home, and so silly things, such as expiry dates, could be resolved with enough time to fix anything that wasn't correct, well before getting to LHR. We are in happier times and the benefits fo VeriFLY are less now, but still the vaccination rules for Canada are different to the UK, and still people turn up for LHR-YYZ flights with incorrect documents.