A “spell effect” is a type of thing in mtg. Its saying the enchantment affects the spell itself not the effect of the spell. I can’t think of an example where that would make a difference but if there is an MTG player will exploit it.

The closest thing I can think of is some copy spells append wording and if it appends multiple lines of text to another spell “effect” wouldn’t stop it in certain situations that “affect” would.