It is unclear what God's purpose with creating the first humans was. It is well known that the humans were once capable of creating and altering the world around them in much the same way as thearch angels, but that power was since taken away from them. According to creation myths the humans simply did not live up to God's expectations.
It is a common speculation that the humans were supposed to continue shaping the universe where God left off. The reason why is, again, unknown. At some point God simply stopped interfering with the world. Some think that God no longer exists or is at least resting, and therefore needed someone to continue in Its wake. Others think God eventually realized that as an almighty, eternal entity, It could never understand the mortal creatures it had created itself, and so It planted the humans as a fairer group of rulers to control all the mortals.
While the humans had their original powers of creation they created a number of other worlds and souled species. Because the humans were later deemed unfit as creators, these species are still often met with prejudice and racism and considered inferior to species created by God.
Humankind's background and involvement with the creation of both Heaven and Hell as they are known today have been the basis for very mixed opinions on their species. Officially Heaven holds them in high regard. This is perhaps most obvious from the high number of humans that have had a seat as a high councilor. However, just as many people meet humans with the same prejudice as the species they originally created.
Humans that are still alive on Earth as mortals are not allowed to know of their history or the powers they still possess. In fact Earth is one of the most heavily guarded worlds in the universe. Not only would it ruin the point of mortality if the humans knew they were powerful creatures, it would also be fatal for Heaven if demons were ever able to enter Earth and once again get a hold on the lineage that allowed them to become natural magic users. No living, mortal human has the knowledge to use their powers.
Even after their original powers were taken away, humans are still the only creatures apart from the arch angels that are supposed to be able to use magic. While other creatures can learn to use magic, only humans can do so without side effects.
Apart from typical magic that can be used to control the surroundings, humans have a leftover ability from their days as creators. They are still capable of learning to create living things. It is an incomplete power that is very hard to use, however. They can not grant what they create a soul, if they can even get as far as creating a functional living creature. Most humans that manage to learn this stick to creating smaller animals that they are very familiar with.
Creating a living creature can not be compared to the summoning magic that some demons specialize in. Summons are mindless compressions of magic that must be controlled entirely by the caster. A created creature is a being in its own right, here to stay, and as such humans must be careful about what they create and where.