E you can just use as shield or damage whenever either is valuable but for W there's a more helpful rule: use W on enemies. Some games there will be a katarina or something to save it for but even in general you want to use W to poly people.

The reason is just that the cc is really strong. Also people die so quickly in current league, typical games have 0 or maaaaaybe 1 big meatball champ who is actually tanky but doesn't do much damage and isn't important to cc. So anyone cced in range of allies will just die. And if an enemy champ is in your poly range but not in range of any allies, your positioning is probably not great. Ofc in some situations eg chasing someone you'll W an ally, but if there's a choice for how to play a fight you basically always want to choose to W the enemy champ who comes in range of you and your teammates.