It creates a lot of interesting decisions for players. The relatively long cooldown means you can use it to catch up with someone running away or run away yourself, but you can't do that every fight and need to make hard (but quick!) decisions about whether now is the time to burn Flash.

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A secondary consideration, that ties into the first, is that Flash has a ton of situational uses that become more and more exposed the more games you play. For example, using Flash to get over a wall is something that is easy to screw up, but pretty sweet when you do it correctly. We want mastery of League to be something that takes a life time, and correct use of Flash is one of those things that you nearly always learn some new trick in how you use it.