Consider childbirth friends, when an infant is delivered into this world it cries, it cries to be contained in the physical, that is to say, that it is the spirit that cries through the physical instrument. For in these fleeting moments the spirit still retains the consciousness of what was before the descent into the physical body and world. But due to the newness and limitations of the physical body it now inhabits it cannot express this.
This is not the only reason friends indeed there are others. Amongst these is the agreement of the forgetting process, the drawing/ draping of the veil across ones recognition of innate knowledge of what truely is. But these limitations are a necessary step in the pursural of growth. This is akin to a human incarnating into a snail. Think of the difficulties and challenges one might face in a lifetime as this creature.
When one returns to human form we might say, after the life cycle of the snail has ended with full memories of that incarnation; one has a greater understanding and appreciation for the freedom and abilities that the human form provides, that the snail form does not.
Can we then say if the entity within the within human form was able to incarnate over and over into various animal, insectoid and plant forms and at the end of each lifecycle, returned to the human form with full awareness of the previous incarnation; would this human entity not have a very great awareness and understanding of the myriad ways in which creation manifests itself uniquely?
Now expand this thinking to outside of your earth sphere. On each uncountable planet, within each uncountable solar system, within the unfathomable arrays of galaxies contained in this universe; there may be a diversity of lifeforms which the human mind may not even concieve. Each a unique part of creation expressed in a different way. Each with its own limitations and freedoms. Each to be experienced by the human entity as part of its understanding of the nature of Creation.
And so this is the nature of the spirit incarnating into many forms including many, that the human entity would be as a snail to. All for the purpose, of experiencing various degrees of freedom and limitation to understand both the nature of creation, and the nature of itself better. By the understanding of the spirit through the various experiences of the incarnation process, the creator understands itself.
For the spirit is a part of the creator, the creator in miniature one might say and provides a feedback loop of all it experiences to the creator. When one considers the implications of each unique experience available and provided to the creator via the spirit is one not truly humbled to have even this limited understanding of the creators plan?