Weapon unlocks, some vanity items, weapon upgrades, the auction house, cosmetic changes, and some undisclosed elements all require Marks of Favour. Aurum, meanwhile, is used for unlocking inventory slots, character slots, progression boosts, bank space, other vanity items, and so forth. Allowing you to exchange between the two means you can always get more Aurum without necessarily spending money, although only time (and player purchases) will tell how generous the exchange rate is.
Secret World Legends players is that you won’t need to pay money for the game. You won’t even need to pay money for things in the game that would otherwise cost money. The game offered a new post today detailing the free-to-play model, which will allow players to exchange Aurum (the microtransaction currency) for Marks of Favour (earned through daily challenges and normal play) and vice-versa. So if you never want to drop a cent, you can avoid it.
Source: Official Site