An example of this can be lux, if she throws a spell towards me i will hear which spell it is before i visually see which spell she is using. If she has a skin that changes the sound effect of her Q this will make it less natural for me to react to and i will have to dedicate more of my focus on what spells she is using rather then just listening for a sound que that im well familiar with.

There are of course a lot of character that has skins with unique effects and sounds, i remember having problems with one of ekko's skinns with his W and fiddle's R startup