Knives are some of the rarest drops in the game. Knives are also extremely sought after by the community. The cheapest knives are around 36 dollars. Each knife has their own design followed by their own skin. Depending on the knife, the skin and its condition/float, it will determine the demand and the price of the knife. Remember that knives are extremely rare and thats why some knives are worth hundreds even thousands of dollars.
Knives do not give a player an advantage over other players. These are purely for aesthetic and bragging purposes. Many collectors try to get some of the best knives out there. There was a StatTrak, Factory New, Karambit- Case Hardened Blue Gem that was sold to a collector for $15,000 USD. Recently there was a StatTrak, Factory New, Karambit- Crimson Web that sold for $30,000 USD. There are only 3 of those knives ever to drop and it has a really good Webbing (The pattern of this skin is randomized).