Actually, it’s not quite nothing, though I understand your meaning. Parry a big sword with a dagger, you may not stop the blade, but you prevent a special effect. Granted, it may not make much of a difference (remove limb is a very strong “special effect”) but it can deny him an Impale or a Bleed or some other definite fight ender. That and he may just roll bad on damage.
I tend to view it as a diverting the weapon AND a side step.
Other thing is, with missiles and shields, it’s almost certainly a hard block with smaller ones.
Consider this option - you aren’t all that good with your combat style (say, 50%) using a dagger and a shield. He swings. A possible smart move is to passive Block with your shield, covering many locations, and then parry with your dagger. Sure, it won’t block damage, but it could prevent him from choosing location around your shield.
Big old shield blocks a lot of damage. Big 4 location shield can block 60% of the numbers possible to hit. Do it the other way around, you are only blocking it 50% of the time, and if you fail, he’s picking whatever location he likes, preferably the ones without armor. Fail with the dagger, he’s picking a location that is not warded, but you may have some armor on there.
Ill be honest, the passive ward with the shield and parry with the weapon didn’t even occur to me until earlier this year and someone else mentioned it. But it’s a fair advantage.