It is because a horse 4 yrs and over would be expected to also be doing ridden classes and therefore being shown in a double or pelham. Under 4 and they are shown with an inhand bridle.
I've been told to and not to by different people! those who said to do so said it was correct for ridden so should be used in hand and others said no-way show in a snaffle!
Show one of ours in hand in a double, so probably answering pretty much the same question Two reasons on her case, she's very strong in a snaffle so would let herself down; and she's won plenty of classes so if she was being shown under saddle she'd be out of novice and it'd be correct to show her in two reins. Suits her head well too. Show others in the same classes as she does (broodmare) in white rope halters because we don't need the brakes and sets the head off nicely. Incidentally if mare is in a white halter, foal is in a white halter too, while foals at foot to the mare in the double are in a little leather in hand halter with pretty browband to match her.
Just to add - over 4 doesn't automatically mean a double, in my little corner of the showing world (and I know naff all about the rest of it), horses in novice classes are generally in snaffles. Once they're in Open classes, it's correct to show in 2 reins.