If I were to take a guess:
- Personal housing, when it comes out
- Dibs on the first set of crafted gears for the next tier of endgame content
- World/server first stuff
When something new comes up, demand for certain items go up, and so prices go up as well. This gives the mercs the buffer they need to achieve their goals. The cycle repeats again when they have the new stuff down on farm, and are willing to sell runs, to prepare for the next set of content. Those who were world/server first Second Coil clears in the current patch would most likely have fully melded i90 gear, which even at this point of time would be expensive and difficult to make.
Gil isn't super valuable in this game, but it has its uses.
