I think you have asked one of the most important core questions of magical practice. The textbook Chaote answer would be that using spirits or not using them are simply different but equivalent ways of doing magic, similar to, say, cursive vs. block calligraphy. While there is certainly truth hidden in that somewhere, I do not entirely agree with it.
So, first of all, there is the question of what we consider imaginal entities to "objectively be". I personally am a strong anti-solipsist, it is my personal gnosis and ethical standpoint to grant strong personhood to imaginal beings, as well as other humans. You will hear opinions of the imaginal being a reflection of one's (or of our collective) psychology, or a "humanized" way to understand the universe. Now I can step in as a crazed Zos-Kia cultist that I am, and claim all of these can be true at the same time, on account of reality being a glorious mess. (Imaginal is a beautiful word I was familiarized with by J. F. Martell and Phil Ford; it means the part of reality that is pure image; ie. image as substance, compare "made of smokeless fire". I have come to love this word quite a lot.)
Anyway, my answer to why working with imaginal beings can be meaningful though, is entirely independent of what you think they "objectively are".
My understanding is that the core issue with pure Intention in magical practice is that our minds are overly complicated and contradictory; if you practice hypnosis for long enough, you'll get a deep sense for just how much this is true... Then, there is the topic of the world being overly complicated and contradictory. In my practice and understanding, magical work (similarly to hypnotic suggestion) is mostly about building consensus. In the mind, and in the world, equally. The ideal you want to reach is "the world" also wanting the result that you want. Working with ancestors, deities, angels or demons allows you to connect yourself to a greater chain of Intention. It's no longer just you standing there, but You as an integral part of the Whole, in dialogue, as opposed to simply shouting demands into the void.
Then again, there are of course other valid ways of connecting yourself to the Whole. Maybe not equivalent, but equally workable.