Hi Harumi!
Increasing jincan gu levels increases the damage. A long time ago the devs put an increasing damage formula however they did not bother to add it to the skill description - so many people wrongly assume increasing jincan lvls = waste SP.
Old old old patch note:
Needle Blow cooldown changed from 25s to 20s.
Needle Blow damage changed from 194.2 + 40.9 per skill level to 254.2 + 56.9 per skill level.
Needle Blow poison damage frequency changed from 0.8s to 0.5s.
Wugong Gu cooldown changed from 30s to 25s.
Wugong Gu damage changed from 252.8 + 73.9 per skill level to 302.8 + 88.9 per skill level.
Wugong Gu poison damage frequency changed from 0.8s to 0.5s.
Throw Gu Pot damage changed from 204.2 + 41.8 per skill level to 414.2 + 91.8 per skill level.
Jincan Gu damage changed from 393.7 + 85.1 per skill level to 593.7 + 145.1 per skill level.