>>96175017
Because in Warhammer, the knights are described as a big part of the Empire, with a lot of political power,, but that's all. They get some lore, you get some flavour option like "if they're blazing sun, they get magical attack, if they're from the inner circle, they get +1 strength but become Rare".
But with the Warhammer RPG, you can pick a single guy in your army and make him his own character, decide what knighthood order he belongs to, what weapons he's used to to, if he has a flair for politics or not, how patient he is with his squire and servants, if he's a philosopher or a practical man...
It's hard to explain properly, but I love the sense of scale that Warhammer Fantasy and Warhammer RPG have, despite taking places in the same world. A soldier in Warhammer is just a 3 strength, 3 toughness, 4 weapon skill man with a sword and shield and some very short blurb about swordsmen being seen as romantic lovers and excellent fencers, but come the RPG and that same non-descript guy can be a rat catcher who decided to join the army after seeing his dog get stabbed by a giant manrat, and who surprisingly found out he was pretty good with a sword, and he might be on his way to become a Greatsword.

I love that the RPG is essentially "pick one of the mooks from your army, and make him the main character of his own story".
And I also love that, in Fantasy, a Knight is nothing to sneeze at. He's just a man in plate armor, in a setting where the only thing weaker than a man is a goblin or a skavenslave, but in the RPG, a knight is a killing machine carrying kilos of steel on his entire person, meaning unless you have specialized weapons, nothing you do will make a dent on him, and he's very very likely to kick your ass back to Morr.
I love that it makes the Empire soldiers feel actually threatening, instead of being just corpses that you throw at the enemy.