「でございます」 is super polite version of 「です」.
Also, to indicate the location of something, です means same thing as にあります。
So she could say 一階です or 一階にあります or 一階でございます.
However, in this case, the one who is answering is department store clerk and the one who is asking is customer. That's why she uses でございます instead of です or にあります。