Blinkers shut down the big screen of visibility, reduce the reaction from the horse to uncontrollable features in the environment and to unconscious body signals you may give him. The horse is designed to notice these detail things, his survival depends on it, so blinkers close down the vision area allowed.
Then consider that a well-trained driving horse understands and responds appropriately to whip cues. The blinkers prevent the horse from seeing the driving whip and anticipating the whip cue. This is important for any driving horse but especially so for multiples. You don't want to be aiming a whip cue at one horse and having the other(s) see it coming and react when they're not the intended target of the cue.
Riding and driving are very different disciplines. Hooking a wheeled contraption to a horse or horses is a far riskier endeavor than climbing on a horse's back, both for the driver and passengers and for all the innocent bystanders and their property who stand in harm's way when there's a runaway horse and carriage. Blinkers (blinders) have been used for hundreds of years and are used by the most experienced and skilled drivers.
The preference of light harness horse instructors and professionals is to prefer a driving horse to focus on what is happening in front and when you commence driving it can be problematic not having blinkers. And I'd never drive one without. Also dependent on what you intend to do, you may find you have to wear get him in blinkers because of rules. The rules exist BECAUSE it's safer. Whenever folks mention in postings about blinkers on BB's someone always turns up to say they've done it or know someone who does/has. But it's not right and it's not what you should do and it's not what professionals do.
I've heard of the occasional (rare) horse who for one reason or another goes better in an open bridle. But IME its more a case that sometimes people just get warm and fuzzy about letting the horse see everything and the results can be disastrous.
I've known folks who went "open" bridle to drive. All claimed to be experienced drivers and with solid driving horses. It worked for a while. In every instance, horse later saw something, reacted, ended up causing a wreck. Some were modest wrecks, others quite horrific. I don't know any of the wrecked horses who were able to be SAFELY driven again, even in blinkers.