Used to be done almost to prescription before racing/travelling. Some still do it regularly, and most have a pre-race veterinary treatment regime routinely.
Quite a few years ago I asked my vet to come and saline a traveller before heading to [ I think ] Dunedin. He was a personal friend as well as our practitioner at the time.
I'll do it, he said, if that's what you want. But your horse has normal electrolyte levels as evidenced by his recent blood test. If salined, he will then have to excrete it, as he doesn't need it. You can't be better than normal, he explained, grinning. They don't have a reservoir for excess fluid/electrolytes just in case!
But, of course, administering these pre-race preparations mean a good and consistent income for vets. They aren't going to refuse the work. But it must add enormous cost overall for the owners. Before someone jumps on me, I'm not anti-vet. Not at all. I trained briefly with that career in mind, but life takes turns that aren't always anticipated so I do what I do, with very moderate success, but love it nonetheless.
Administering a drench post-race is much more sensible, to help the animal re-hydrate after racing and travelling. Or just do a Myers and toss them in a grass paddock for a few days. They'll re-balance themselves.