Glacial is a versatile rune, and fits many different scenarios and gameplay patterns. Whatever your playstyle might be, Glacial Augment can fit in (with varying degrees of success of course)

Phase Rush is for kiting/repositioning/escaping? Well, Glacial Augment can do that too. While it may not grant you the movespeed and slow resistance, it still does allow for kiting, by slowing all nearby enemies for a ridiculous amount. It allows for repositioning in the same manner, the enemies can't easily stop you from repositioning if they can't actually reach you themselves. Escaping too

Glacial Augmentis nowhere near as powerful of an early game tool as Electrocute, but it can still provide you with tools to play more aggro during the lane. The slowing auto attack is great combined with cheap shot, for some easy AA + Q harass, and it allows you to hit empowered Q's even easier than any other rune. Into certain matchups, the slowing AA is of course not very useful, but most of the time, you don't wish to have lane presence in those matchups either way, here you'd rather just sit back and scale.

Furthermore, if Predatoris the best at chasing people down and flanking, Glacial Augment is right on its tail in second place. The slowing from Twin Shadows is possibly one of the best chasing tools in the game, and it allows for easy picks around the map, even through scarce vision. Even if one skips out on Twin Shadows, GLP allows for flanks of the same caliber. Slowing an entire enemy team down in a "no movespeed allowed" zone, is detrimental both to the enemies teamfighting capabilities, as well as their mental state.

As such, while Glacial Augment is not the best rune at anything, it is great at a lot of different things, allowing you to adapt to the current game you're playing.