Stridebreaker and Randuin's Omen can proc Glacial Augment very frequently. Glacial Augment also makes auto attacks slow enemies, especially if it's a melee attack. Except the attack from Poppy's passive, Iron Ambassador, is long ranged and counts as a melee attack. This can help you slow enemies in the early game and when your items aren't up, all from a safe distance. So, Glacial Augment in general allows you to land your abilities much more often and stick to your enemy.

Prototype: Omnistone has a much faster rotation for melee champions as it is often used for ranged champions who could easily use every rune. But Poppy is a melee champion that can use ranged auto attacks, so she is more effective while getting runes like Press the Attack. Poppy can also use pretty much every keystone pretty well, and can appreciate Inspiration as her main tree.