So like, why can Annie perma-hold her stun?
Pretty much every other major "charge-up" ability (Jhin's fourth shot, Yasuo's tornado/knockup, TF's stun card, so on and so forth) have a limited duration to which they can hold their ability, as to not be oppressive to the enemy laner.
So why can Annie permanently hold her stun and just farm with autos if she wants? Them coming in range of her Q is asking for automatic stun + burst + TLD proc poke which will do a fair bit of damage. While all she has to do is just continue farming.