"Show in address list" option in creating new address

I have no idea but my guess would be that somebody (or many people) who has a huge number of address objects but only use address groups in the policy didn't want to have to scroll all the way down to get to address group list to select one of them (on the right side pane in a policy creation GUI) when a new policy needs to be created. And that user/users happened to be a big customer of Fortinet products (Therefore has a big influence).

My guess might be off the mark but I wouldn't expect the real story is not far away from there.