What is the benefit of having an "Inflict X damage to your opponent" spell card that does less damage than others?
There is no benefit whatsoever. The only reason why someone would use this sort or not-as-useful card is that there are some decks that mainly focus on damage from spell or trap cards, such as 'Magic Cylinder' These decks basically only contain cards that deal instant damage. It is one of the most quickest way to deplete someone's LP. The benefit is that not many decks are prepared for this type of damage, and since there are only three copies of a single card within a single deck, varieties of a card are necessary.