Why all those verb forms in Greek?

From a book I'm editing: Why are there seven verb-forms in the indicative mood and only three in the nonindicative moods? The answer is simple: the forms in the indica¬tive mark both aspect and tense (the future forms may mark only tense and not aspect); outside of the indicative they mark only aspect. Since there are only three aspects, there are only three verb-forms outside of the indicative.