Some people need to understand when to complete an Item

Cinder is an item I think now in the meta for Tank top lane is needed in order to trade and do well, but if your playing vs a Mage top lane you don't want to complete this item right away. You can get it but you don't have to complete it first item. Instead just go straight for a Cowl and finish it into a

Same with mid if you're playing an ad vs a Mage, you want to go with a but not having to complete it

Why can't people just understand when and when not to complete an item. I know a lot of High elo player tend to tell player they teach how to finish an item first before moving on to a next item build path but knowing how to just get half a certain item of something is enough and can be completed later on when you need it to