Five inputs are observable. Price and strike are on the screen, time is a calendar question, rates come from the money market, and dividends are forecastable within a few cents. Volatility is the only one nobody can look up, which is why it absorbs every disagreement in the market.
This is why quoting in volatility terms is standard on trading desks. Two traders arguing about whether a call is worth $2.30 or $2.42 are really arguing about whether the next 45 days will be 25% or 26.3% volatile, and the second framing is the one that transfers across strikes and expirations.
Example: XYZ at $50, $50 strike, 45 days, 4% rate, no dividend before expiry. Feed 31% volatility and you get $2.29; feed 35% and you get $2.57. Everything else stayed fixed, which is the point.
Related: implied-volatility, rho, implied-dividend, black-scholes-model