Basically it is for appearance, design, workmanship, better materials for more durability, and possibly functionality. Some of the high-end designer tanks are styled to look particulary great on certain PV's. Examples of these are the Vaporfexion Pro-V line, the Kir Fanis Prive line, and the ZAP rba all designed with the Provari in mind.
There often is no comparison of the build quality and durability between a high end designer tank and the economy tanks made by Smoktech or others in that price range. Heavier and more finely machined materials such as stainless steel, anodized aluminum, or titanium make the tank more durable. More craftmanship is often involved.
The tanks made by Hoosier Vapes, Unique, and Jonboy USMC Lava tanks are often one-of-a-kind works of art, and are unique in function because of the special designed top cap which can be twisted off for juice easy filling.
Some people like decorative tanks as an expression of their own personality, and makes them feel good about themselves. Some people become collectors.
All told, they all pretty much do the same thing, and that is to act as a reservoir of e-liquid for the benefit for the real workhorse in producing vapor - the cartomizer.
I continue to be amazed however by the high end tank makers who even at this time are slapping cheap polycarbonate plastic tubes on their very expensive mods, when a few more dollars in expense they could offer pyrex or quartz glass tubes that wouldn't melt and render the tanks useless when the owner unwittingly puts some cinnamon or citrus flavor in their tanks. Tanks that pretty deserve to wear pyrex.
Above are some examples of high end tanks: Cirrus pyrex tank on a Provari, Hoosier Vapes Bella and Elegance tanks with a custom hand-blown High Desert tank, Phiniac pyrex tanks with sand-blasted designs, and a Vaporfexion Pro-V tank on a Provari.
