The Library episode

I teach English in Japan. One of my students loves Seinfeld, and always has questions about stuff from it. Today she asked me about the episode The Library...

Here is the quote she asked about...

"Yeah, 71. That was my first year on the job. Bad year for libraries. Bad year for America. Hippies burning library cards, Abby Hoffman telling everybody to steal books. I dont' judge a man by the length of his hair or the kind of music he listens to. Rock was never my bag. But you put on a pair of shoes when you walk into the New York Public Library, man."

Bookman, the library officer, says this. What did he mean by "you put on a pair of shoes when you walk into the NY library"? I can't quite remember the episode... I couldn't think of a way to explain it to my student. Any help would be appreciated. Thanks! :D