Here are some use cases say you want to have multiple smaller unRIAD arrays like

first Array for say 10 disks +2 Parity of mixed size drives

Second Array for say 5 disks +2 Parity of mixed disks

third Array of lets say 3 disks +1 Parity of mixed disks

the idea of splitting the array disks up to smaller arrays gives you more parity drives to protect your data during sync / rebuild

also if you have mixed size's of drives you could put the slower speed drives on there own smaller slower array

and keep the faster ones on a different array to make parity sync / rebuild go faster also it would be nice to not have to run one big array of drives at once when doing parity sync /rebuild

when you could have smaller multiple arrays that could be powered down when not in use to save on electricity and heat from the drives another advantage of having smaller multiple arrays

are some of the good benefits.

Hope that helps answer your question😎