In this game, they act as a hub where players can pick up letters and special items that have been delivered to their account
Almost.
With a radically different card frame to normal, and proving to be one of the game's most powerful mechanics in a long time, Adventure can be a tricky one for newcomers to understand
What Are White Decks Good At
It's most often played in decks that can ramp into it quickly, most notably Eldrazi Breach decks in Modern, and pops up in niche Sneak and Show builds in Legacy
While it might be tempting to try to buy the Fires Friends list and drop the planeswalkers to play Jeskai Fires (which is a top-tier deck in Standard), apart from Fires of Invention, Kenrith, the Returned King, Deafening Clarion, and one copy of Steam Vents, none of the cards from the Challenger Deck actually show up in Jeskai Fires