One point is that the efficient cause, on which you concentrate, needs something to operate on, something to apply to. The form determines or limits what the efficient cause can produce. How in any case could the efficient cause operate on something formless ? But this is the short answer and we need to take a longer road.
▻ THE METAPHYSICAL SETTING
At its deepest level, Aristotle tells us in Metaphysics, A, 1, 981b28, philosophy looks for 'the first causes and the principles of things' (J. Barnes, 'The Complete Works of Aristotle', 2, Princeton : Princeton University Press, 1984, p.1553). To know these causes and principles is to have wisdom (sophia).
Metaphysics, Δ, 1, 1013a17 (Barnes, 1599) identifies causes with principles,
so we can just talk about 'causes' (aitiai) from now on. The divergence between aitiai and causes will emerge as we go on.
▻ THE FOUR CAUSES
In Metaphysics Δ, 2, 1013a-1014a and Physics II, 194b-195b, Aristotle distinguishes four causes : the material cause (to hupokeimenon), the formal cause (to to me einai), the efficient cause (arche tes metabolis) or cause of change, and the final cause (to telos, ou heneka) or that for the sake of which.
▻ THE MATERIAL CAUSE IS NOT WHAT IT SEEMS
'Matter' (hule) for Aristotle is that out of which a thing becomes. So while 'matter' may be the bronze from which a statue is made, it may also be the seed from which a plant grows, the premises of a conclusion, the letters from which a word is formed. The hallmark of matter is that it is something potential, the dunamis or possibility of being formed into something. It is always incomplete or what is not yet - something passive which requires determination into something specific.
▻ THE FORMAL CAUSE
This determination, or fixing of specificity, is the formal cause. Aristotle uses a variety of phrases for this : to to me einai (as above), morphoi, logos, eidos, paradeigma. Form relates to matter as that which determines the passive into something determinate (or more determinate). Thus the shape of a statue which the bronze is transformed into, the word into which the letters are arranged, the whole which combines elements into parts, the law which regulates the behaviour of objects, the specific difference which distinguishes a species within a genus : all these are formal causes. The formal cause is what makes the determinable determinate.
▻ THE FORMAL CAUSE AS ESSENCE OR ESSENTIAL NATURE
The formal cause of X is the essence or essential nature of X, its essential property where a property belongs essentially to X only if X would cease to exist without that property. The identification of formal cause with essence is clear from an abundance of texts : Metaphysics Z.7, 1032b2, Phyics 2, 194a21, 3, 194b26, de Anima 1, 412a20. An essential property, hence a final cause, is also explanatory.
There is also a link with definition. When one has identified the formal cause of something and pinpointed its essential properties, one also can give its definition.
One obvious application is to the question of identity. Someone, Y, a human being, can change in many ways over time. S/he can grow, lose their job, change occupation, contract a disease, become the citizen of a different state, be reduced to poverty, have a limb amputated but still remain a human being and the same human being. But if s/he ceases to be a living organism, this is an essential property and losing it means ceasing to exist as a human being.
Being a living organism also fulfils an explanatory role in relation to other properties that Y has. If Y is a human being, then ceasing to be a living organism entails losing all the properties that belong to Y as a member of the species, human being.
▻ THE FORMAL CAUSE IS NON-TRIVIAL
If we follow the exposition above, it is clear that whatever difficulties attend the concept of a formal cause, triviality is not among them.