I hope that design concept gave Mr Yamasaki and future tenants a sense of security.
The design idea of a super strong 'exo-skeleton' of massive steel columns would have been reinforced by the narrow spacing, apart from the aesthetic and safe-feeling consideration cited by Shorpy-ite Michael R.

According to architectural lore, Yamasaki was afraid of heights. He especially did not like to stand in front of wide plate glass windows which were broader than his outstretched arms. He designed the World Trade Center towers with their exterior steel columns placed 3'-4" on center, with a gap of only 26 inches for the windows, so that the buildings' occupants could avoid the feeling that they might fall between them. That's why I find his stance in this photograph to be quite revealing.