The Saddlebag adds a second tab to your bag with 200 extra slots. Permanent, from the moment you own it.
Why: loot piles up. Uniques you're comparing, sets for a second formation, mythics from the Abyss waiting for the right hero. That doubles the room of the base bag.
What it does not do: it adds space, nothing else. No stats, no drop rates, no power.
Achievements are not affected: every achievement in Idle Legions can be earned without this DLC.