A Summoner event, its major strength is to make Celeste the ultimate summoner killer. It makes it very hard for a summoner to guard from Celeste, and forces the opponent to spend a lot of gates and units just to completely surround their summoner in the hope it is enough to prevent Celeste from reaching it. That’s why it’s important to poke opponent’s summoners, because once they are low enough life, they will be forced to spend a lot of resources, time and effort to protect their summoner against the looming threat of this card, which Wayfarers can capitalize on to get an economic lead with better trades.  Just the threat of Slip By existing is the real power here.

In a pinch it can also be used to escape a bad situation, allowing Celeste to safely target a high value target like an opponent champion or just be able to use Celeste's strength a bit more freely and safely. It can also be used to quickly advance Celeste to a position to build a gate or an Astral Waypoint offensively. 
However, the card and its magic cost make it situational for those uses as the Wayfarers' economy takes a decent hit in exchange for those actions and while that economic hit is irrelevant if your opponent’s summoner is dead, it is quite relevant in all other scenarios.