It seems to be presented as something unusual. Some early sources say that it was a garment of the poor (it could be woven by a person who had no particular skill). Others see it as symbolic, perhaps hearkening back to the special "coat of many colors" worn by Joseph in the Old Testament, or the garment of the high priest.
Perhaps most plausible is that John sees the tunic as symbolizing the unity stressed so often by Jesus. For example, "so that they all may be one, as your, Father, are in me and I in you." (John 17:21)
- Little Black Book, Diocese of Saginaw