Kindly note that the Legend Pass unlocks exclusive rewards for you in the Season Pass event, among which you will be able to collect exclusive bonuses that are available only within these rewards. You still need to complete the season missions to progress through the season tiers and collect all the rewards.
Also note that when you buy the Legend Pass, you get a +10% Trade Coin and Reputation Point boost immediately, and you will also be able to collect a +50% point boost while unlocking new tiers in the Season Pass event.