The particle "に" can be used in a variety of ways, in this case, it refers to the place where a thing or person exists or appears. As you know that the particle "が" is Identifier particle. case 1 1。駅の近くにポストがあります Place is 駅の近く(near thd station) What we are talking about is ポスト(the post). case 2 2。花屋と本屋と美術館の間にあります。 Place is 花屋と本屋と美術館の間(Between the flower shop, the bookstore and the museum) What we are talking about is unknown in this example (because it is abbreviated), but we are talking about something that exists. Why it is abbreviated, because we know what we're talking about. If you want to use "が", 花屋と本屋と美術館の間が、それのある所です。 What we are talking about is the place where between the flower shop, the bookstore and the museum.