On the film's website, the director, Gary Hustwit, does a good job of justifying why he made a film about a typeface:
Why make a film about a typeface, let alone a feature documentary film about Helvetica? Because it's all around us. You've probably already seen Helvetica several times today. It might have told you which subway platform you needed, or tried to sell you investment services or vacation getaways in the ads in your morning paper. Maybe it gave you the latest headlines on television, or let you know whether to 'push' or 'pull' to open your office door.
Given the discussions I've occasionally participated in about what font editors prefer on manuscripts, I figured that there must be some readers out there who are as intrigued by this film's premise as I am. And hey, they interview someone who was a year or two behind me in high school, so it must be good.