You need to know what champion you are playing and what role you need to play. Sometimes the suggested items don't fit the play style you are trying to play. An example of this is Gragas, I play him as an AP mage, however the suggested items are tanky items like doran's shield. One way to see what items champions benefit from is looking up guides on how to play heroes on the internetwebsites such as mobafire or leaguecraft.
However building items to fit the siutation you are in and what heroes you're against is the most appropriate way of playing.