I’ve seen it don cross country schooling, especially for horses not far from their race days. It takes the edge off so the rider can focus on introducing the horse to cross country jumps rather than dealing with them losing their minds with all the horses running around.
It is also used in some horses for the first few rides, even in an indoor, for the same reason.
It is used in some horses in the winter when even though they are on 24/7 turnout, they aren’t moving much and snow/ice sliding off the roof of the indoor is scary, etc.
Some racehorses get it when training because they get to wild but on race day, they don’t but they are more focused on getting to the head of the pack - rather than training days where they aren’t running in a group like that.
Some fox hunters get it before fox hunting day so they aren’t to wild.
I used to be more against ace (which is what I’m used to using) for riding until I started encountering some of the above scenarios and found it really did allow for training progression a lot better/quicker than not having it. I would not offer it as a blanket solution but do see some benefits now, for certain horses.