Haibane Renmei
Theories

The series leaves us all with many questions about the Haibane and Glie.  It may purposely leave many questions open in the air, or it my be that nobody has the answers at all.  That has led me to think about the Haibane on many occasions.  It leaves you curious to the Day of Flight, what significance life of the Haibane has there, and many more questions.  I will take the time to answer in my own opinion some of these questions.

Contains spoilers.

The reason the Haibane are here.
This seems like a logical place to start.  I believe it is so that they can learn the lessons that they need to learn to continue living, or to pass on.  Many Haibane leave the town after a few years there, but there are some that haven't.  And they live their days as hermits on the outskirts of the town, or so the Communicator says.  Many Haibane learn valuable lessons before they leave.  Reki's lesson nearly cost her life, and the life of Rakka too.  Reki's only chance to ever leave was to learn to ask for the help of others, when she would never ask anybody.  More proof is the the rules that the Haibane must follow.  One example is that they must work.  It teaches them responsibility.  The second is that they may never own money or anything new.  This makes them realize how full life can be without the need for fancy toys and gizmos.  They began to find joy in the simple things they do, and in each other.

The Town Of Glie
This town, that nobody can pass has walls that could easily kill any mortal, is protecting the inside world from the outside.  It's a tough love because only he birds and Toga have experienced what lies beyond.  So what lies beyond the walls?  I believe nothing does.  Now don't get me wrong, I'm not saying the Toga are nothing but wandering dreams, I'm saying that the town lies in between the realm of the living and the realm of the dead.  I believe it is a passing point where some spirits get a second chance at life before death, and where others find their path back to life.

Why there are humans in the walls
I don't think that this needs much of an explanation, but it's still a theory.  Imagine a city without a constant population.  One where it's residents can disappear at a moment's notice.  They may not even come and go for years at a time, but they all go.  That is what it would be like if only Haibane roamed this world.  Humans add the stability that makes it able for the Haibane to grow as a person, without fear of the unknown.  In this way they protect the Haibane, making sure that there is always room for Haibane to come and go as needed.  They keep this town from falling apart.

The Toga
The Toga are a mysterious group of travels that wander into this town with many items for barter, but never speak a word.  They talk in a special sign language that only they, the communicator, and Rakka have learned.  I believe the Toga are a group of priests that watch over a town filled with lost souls.  I believe it is their job to keep the town running and drop by to trade as a reason to visit the town and examine it's shape.  The book that comes with the first dvd claims that they may be people who left the town by breaking the rules, and want back in.  All in all they are very mysterious, and you guess is as good as mine.

Why people are named Japanese names in an obviously Eastern European town.
This one was actually the first one that I ever thought of.  Scratching the fact that the creator is Japanese as well as the series, I have come up with only on conclusion.  This town was formed of many cultures when it first appeared and must have had a strong Japanese background for many of it's original members.  Then eastern Europeans moved in and with their architectural leadership the town was built.  The other reason I have thought of was to mix two completely different cultures together would confuse anybody that happened to lose their way into or out of the town, and would help keep it a secret.