If the character expects to regularly receive buffs that apply to a single attack (eg the advantage from Guiding Bolt), having that buff for "all" of their attacks for the round could be more valuable than just getting it for the first of many small attacks.

Similarly, there may be situations where the character can only make a single attack (opportunity attacks, the extra action from haste, etc). If they've somehow optimized for one big hit, they might be able to make that big hit more often.

If the character expects to regularly receive buffs that apply to a single attack (eg the advantage from Guiding Bolt or a Grave Cleric's Path to the Grave), having that buff for "all" of their attacks for the round could be more valuable than just getting it for the first of many small attacks. Similarly, a foe might be under a condition that would grant advantage (or more) to attackers that ends (or gets a new save to end) when the target is hit (eg Sleep or Hypnotic Pattern).

There may be situations where the character can only make a single attack (opportunity attacks, the extra action from haste, etc). If they've somehow optimized for one big hit, they might be able to make that big hit more often.