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Why is she essential for speedruns? That's a cool fact.
Sorry I forgot to respond! It's because of her skill Hired Help. It has a fixed damage output that isn't tied to the strength or level of the character using it, so it's the main way of dealing damage to a boss whose recommended level is 45 when your characters are level 13-ish. So the speedrunner sets up Armor Corrosive (the Thief skill that drops defense), has the boss in Break state, and uses a fully Boosted, max-expense Hired Help, and it does something insane like 27,000 damage. It's so useful the speedruns I've seen always pick up Merchant as a secondary job so that two characters can utilize it. I believe it's the only secondary job picked up in the runs, though I might be mistaken.
Tressa's passives are also useful for the runs. The characters are too low-level to steal anything good, so you need to use Purchase for a few NPC items (mostly Large Spell Stones), and Tressa uncovering money from all those unfortunate souls adds up to fund all of the Hired Helps and Stones needed to get through the speedrun.