Using intent with energy / manifesting with your own abilities has its limits. For most daily living, you are a physical creature bound heavily to the physical world, which has implications on our ability to move and work with subtle energies. We are definitely capable of working with subtle energies, but we must train hard and extensively for this capability. And even after all that training, it is still not second nature to us and we still have limitations with this. Regardless of sigils, manifestation, law of attraction or some energy working method, our ability to realise outcomes through our own power can sometimes be limited unless if we are truly advanced. Speaking from experience, I have found the greatest success with using these methods for internal work and less so for external change.
Regardless of whether you consider them egregores or actual external intelligences, spirits are capable of moving and manipulating subtle energies just as skilfully as we move and manipulate things with our opposable thumb based hands. They are largely made up of these types of energies and so this comes naturally to them. Their ability to move things in the physical world is usually limited, but their ability to move the subtle energies higher up in the chain of manifestation certainly beats ours. I believe they are fully capable of working these energies in complex ways, which we lack when do do things through stuff like sigils, manifestation or energy work.
Like I said, manifestation could be visualised as a chain of events, going from the subtle created ideas to the coarse mundane manifested reality. This chain is implied in several constructs such as the tree of life, even though there are variations in how this chain looks like. Manifesting a simple result requires a simple manipulation of energies in this chain in order to produce a synchronicity in the physical world, so using something like sigil magick can work. But a complex result? This can be more difficult to do and require a more complex manipulation of the subtle parts of this chain. A spirit, especially one suited for the type of energies associated with your result, is best qualified to play with the subtle energies of the chain in order to produce a result that could manifest into the physical world.
The power of spirit based manifestation comes when all parts of the chain do their jobs and play to their strengths. The spirit manipulates the energies in the subtle parts of the chain in order to create the conditions for the manifestation of the result into the physical world. You do everything you can in the physical world to bring the physical world closer to those conditions the spirit has energetically created for you. By working in harmony like this, you create the situation where a complex result manifests. Without the spirit, servitor, or egregore to do the subtle part for you, it will be much harder to enable the manifestation of something complex.
Of course, take what I said as nothing more than speculative theory. To me, it makes sense based on everything I have read in Hermeticism, Kabbalah, etc, but you might have your own takes on it.