It isn’t just for that immediate show. When it was enacted for h/j, it was multipronged. A owner just can’t decided to change the horse’s name, age, start of new hose recording and not notify the USEF on change of ownership/change of horse name. It allows the horse to be searchable via microchip number by the breeder, no matter if the horse changes names or not.
Here is an example: https://www.chronofhorse.com/article/updated-eventer-turned-hunter-raises-usef-record-keeping-questions
And it can’t just be any microchip, it has to be ISO compliant (don’t know what number they are on). They also physically link the horse to the registration/competition records. It also serves as a way for veterinarians or transportation companies to validate identity to ensure that they have the right horse when treating or transporting ( for owners that aren’t there or ship).
Besides being required for some competitions, microchipping is a permanent method of identification that can be used to restore horses that have been lost, such as in fire incidents or natural disasters; to reclaim stolen horses; or to provide proof of ownership in cases of travel and competition. It is already used by many breed registries to provide a record of horse identity and ownership.
I don’t think anyone competing has to go out and microchip their horse if they are not showing modified (or plan to). But there are certainly benefits.