We all make the mistake of developing habits when building items. We overvalue or undervalue certain items, build the same items regardless of lane, etc. While some champions are shown to seem to be at their strongest following certain build paths, it's important to remember that there'll be times we have to deviate from these to be effective against certain team comps. Poppy is this personified. Obviously she's best as a tank, but even for a tank she has a wide range of items to choose from. Need to grind down tanks? Black cleaver. Need to push lane? Sunfire cloak. Need to beat the living **** out of some douchenozzle taking Leblanc Top? Hexdrinker.

Tl;dr With decent base damage and percentage max health damage, she can always adjust to a situation to remain useful.