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Why Two Swords?

I get that in Geralt's world Witchers use 2 swords for humans/beasts and monsters respectively. I'm curious if the author of the Witcher series gave any reason why this is so. It's plausible that steel is not as effective against monsters as silver, perhaps because the latter metal may have some magical properties that make it effective against them. It's not a new plot device. For example in modern books and films it's commonly portrayed that the only way to kill a werewolf is to use a silver weapon; to kill a vampire one must drive a wooden stake through its heart; and so on.. But why can't silver be effective against humans and beasts too? Why is it that steel is preferred when fighting humans and beasts? If ti's toughness of the metal, surely some of the monsters that need a silver blade have natural armour such as thick hides?
I haven't read any of the Witcher novels so I'm wondering if this was explained somewhere in the books. I've played 1 & 3 and this was never made it clear in the games. Not that I recall anyway.