Omiya station is located in Saitama city, Saitama prefecture. The station is called Omiya due to existed in former Omiya city, which merged with Urawa and Iwatsuki to form the current Saitama city. Although the station is the busiest not only in Saitama city but whole Saitama prefecture, it is the not main station of Saitama city. The main station is Urawa station.
Omiya station is served by JR, Tobu railway and Saitama New Urban Transit co. JR operates Tohoku main line(mostly known as Utsunomiya line), Takasaki line, Keihin-Tohoku line, Saikyo line, Kawagoe line and Shinkansen. The Shinkansen are Tohoku(including trains running through Hokkaido, Akita and Yamagata Shinkansen) and Joetsu(Hokuriku Shinkansen which is officially between Takasaki and Kanazawa also runs from Tokyo station via Omiya station).
Omiya, which all Shinkansen trains call, unfortunately does not have ticket office for exchanging Japan Rail Pass. I assume the reason is that Omiya is not a place for foreign tourists to stay. Even Japanese do not overnight in Omiya, which is just 30 minutes from downtown Tokyo. Also, Saitama prefecture is one of the prefectures in Japan which suffers from lack of tourist attractions. However, due to good location Saitama can be a cheaper place to stay during Rugby World Cup 2019 and the Summer Olympic games in 2020.