>>1613619performance reasons (stiffness, lightness). By far the most important performance aspect of a crank is just having a good condition bottom bracket though, regardless of what that is.
durability (the interface which they fit on regardless of system, will wear, as will the extractor system if they have one, pedals threads can strip, chainring bolts especially get fucked up several ways and it's extremely annoying, chainrings themselves wear differently depending on quality
situationally, the actual gearing options they support, that's a big one. Really shit cranks you can't even change the chainrings. The bearings they use. A lot of cheaper cranks and meme cranks use bullshit standards which are hard/expensive to find bearings for, or the bearings suck.
aesthetics
like all things bicycle it's complicated. Or, if you want it to be, simple. If you don't have a problem, you don't have a problem.
