The main significance of the fact that there were multiple species of hominins in East Africa all at the same time is that it makes it harder for us to believe in a narrative in which human evolution is a process of inexorable progress that inevitably led to us.
We often think of human evolution as something that was directed and systematic. We think that one species evolved into another in an orderly way, leading to our own species at the apex of evolution. The presence of the multiple species of hominins implies that the process was much messier and less certain. There were many kinds of proto-humans which could have evolved into something like us. This makes it seem much less as if our evolution was fated.
If multiple species of our ancestors lived at the same time, it forces us to realize that our evolution was not inevitable and that history could have turned out much differently than it did.