Duplicating the key removes some accountability. With this set up you can revoke access to only one person, while leaving the access in place for everyone else. If you had a single lock with six copies then a bad actor getting a copy means you’d have to replace everyone else’s keys

This also means one person can’t take their lock off and replace it with another, and therefore lock out everyone else

One case I've seen this, local PD, Sheriff, city Park District and county Forest Preserve District all needed access. They ech have dozens of hundreds of their own locks keyed the same, so they only need to carry one key, but they all need access to this one gate. If it was only one gate, it would be inconvenient but not too difficult, if it was a dozen shared gates, each with different overlapping agencies and each with their own key, every officer or employee would need a big ball of keys again, defeating the reason behind their keyed alike locks they already have.