Among other things it is used for Swiftness Potion and Thistle Tea. Both of these continue to be useful even at high levels, making swiftthistle by far the most expensive herb in the areas where it grows. It is also rather tedious for a higher level alchemist to gather, so can be a great source of income for an aspiring herbalist who is adventuring in those areas anyway. Due to the Energy and Agility bonuses, it is the single most prized herb for Rogue Alchemists.