Take Summon Monster I as a spell known. If you use a 2nd, 3rd, 5th, or 9th spell slot to cast Summon Monster, you get Summon Monster II, III, V, or IX instead.
Something similar applies with other spells that reference another spell for its effect. Magic Circle vs Evil also allows you to cast vs Good, Law, and Chaos, and so on.
Of course, that gets freakishly good after awhile. Polymorph, anyone?