Free chunk damage, paid for in restraint the other 90% of the game.
Box placement is almost always the better use of mana and time. This is the exception: Comet off cooldown with lane priority is free damage sitting there. Walking longer in Q’s invisibility buys more distance, but E has to land before or right as the invisibility runs out, wait too long and the surprise turns into standing in their auto range with E already spent. Best out of a bush, the approach is already hidden so the whole thing reads as a normal walk until the poke lands. Check the map before committing: E is on cooldown afterward and there’s no slow, so if either laner turns to fight, or a gank is already moving, there’s no follow-up and no way out. Rare on purpose, worth it exactly when it keeps Comet on cooldown instead of wasted.