Whenever someone talks about purity he seems to imply that higher purity should translate in a boost to every single attribute of an item.
I don't think that is absolute necessary. I will go as far as saying that it is not even always desirable.
Say item A has 3 different attributes, like speed, durability, rare drop chance for example.
If purity linearly ups all those 3 then it is no more than a "level" on the item, a purity 30 item is absolutely better than a purity 10. In this case there isn't much variety, there is no reason to use a purity 10 if you can afford a 30, so the main goal of the game easily turns into becoming able to afford the highest purity possible. And that in turn could turn into a grinding situation.
If instead purity affects the various attributes in different ways, so maybe it raises drop chance while lowering durability or the reverse, then you can easily see how:
1) there is more variety in what item A can accomplish, and variety is fun
2) it is likely that you will end up with different versions of item A to use in different situations, and more choices is generally also fun
3) you can't no more compare versions of item A in an absolute way, it is even likely that low purity item A would still remain a desirable choice for most mundane tasks
For example, when talking about durability of Axes, I would have the unpure version be the most durable or the most balanced one. A pure Axe could be a lot faster but also break sooner, or maybe have a higher chance of dropping cross shaped spliters while being slower (maybe even different effects based on purity element).