It lets you estimate the theoretical fair price of options, you can compare with current market price and see whether the option is overpriced or underpriced.
It lets you estimate the time erosion and see how much loss you would incur if you hold the option with you (without any change in the underlying)
It enables you to arrive at a stop loss figure
It generally outlines the payout graph and let you understand when you will reach the breakeven.
It provides the option greeks enabling you see how much the option price will change for every unit change in the underlying. (delta)
Sometimes you get to know the put call parity also, the difference in call and put prices for the same strike price.