Legend is the ‘power stat’ of the game, in much the same way as gnosis is the power stat from Mage.
The higher the character’s Legend the more essence (mana) they have, the more potent the powers they can unlock, and the better they are able to resist supernatural effects.
At Legend 5 the characters literally become demigods, and if they were ever able to increase their Legend to the lofty heights of 9+ they would achieve apotheosis and become gods in their own right.
Legendary characters draw the attention of Fate.
Characters with high Legend live perpetually in ‘interesting times’ as their fateful aura attracts improbable events; for example, if there is going to be an armed robbery, it will happen to happen just as the character goes to the bank. This neatly explains ‘PC syndrome’ in-game.