The paint seed (pattern index) and float cap together create the rare patterns collectors pay a premium for, like low-float Case Hardened blue gems or centered Crimson Web placements.
Two skins with the same wear tier can sell for very different prices purely because of their seed. High-value patterns are tracked by community pattern guides.
For most buyers the float tier is enough; if you are chasing a specific pattern, confirm the seed on the destination marketplace before buying, since pattern is not reflected in the wear tier alone.