The right items on the right unit

Every champion scales with something. Some deal their damage with attacks and want attack damage and attack speed. Others deal it with their ability and want ability power and mana. Put an attack damage item on a spellcaster and most of what you paid for sits unused.

Where the unit stands matters too. A carry in the back line is meant to be protected, so defensive items on it are usually wasted slots. Tanks in front are the opposite: they want armor, magic resist and health, because their job is to stay alive long enough for the carry to work.

Some items do not pick a side. Hybrid items help both kinds of damage dealer, and utility items do jobs that have nothing to do with stats. A few champions break the pattern because their ability works with an unexpected item. When unsure, check what the champion's popular builds use. Those builds come from real games, which is why the game review only calls an item wrong for a champion when it shows up in under 3% of them.

You do not need the perfect build. A good item on the right unit beats a perfect item waiting on the bench.