Some common swords do more damage than the silver sword. I'm level 7 now and don't see why that silver sword is special? Why do I need two swords, when I could just use one of them all the time?
"Steel for men, Silver for monsters. Sadly, I have used both." -Geralt of Rivia.
The silver blade is the most effective against monsters, but does minimal damage to men, elves, and dwarves.
The Steel blade can be uprgraded by meteorite chuncks, and is mainly used for humanoids. During my time playing, I noticed it was also more effective against woodland creatures(Wolves), than the silver sword.
Hope this helps!
It makes fighting monsters easier, because many of the beings that appearing in the conjunction are weak or vulnerable to silver.
It doesn't really make sense that no one else uses this knowledge, other than Witchers - especially because in the world literally around every corner there is a monster of some kind, often almost next to settlements.
You would think at least some people would have silver swords.