What does it mean that only permanents are legal targets for spells or abilities?
Consider a creature like Nectar Faerie. It says


  {B}, {T}: Target Faerie or Elf gains lifelink until end of turn.


Now we know that spells can have lifelink, as per Soulfire Grand Master. 

And we also know that there could be Faerie Tribal spells, such as Faerie Trickery. 

This rule exists to clarify things like... "You could not use Nectar Faerie to give Faerie Trickery lifelink because it doesn't specifically say you can".