Film ages. Over time colors and speed will change. Refrigeration slows down this process, freezing slows it down even more. "Consumer" film is released early, ages through a "premium" point and then ages again. When it reaches this "premium point" is anyone's guess and it depends how it is stored. The difference between fresh, aged and "premium" film is small enough that you probably won't notice the difference. "Pro" film is released at it's "premium point" and so should be cold stored to prevent changes.

NOTE that pro film does not age faster then consumer film, it just starts out at a different point. So if you intend to store your film for years, freeze it. If you intend to store it for a few months, refrigerate it. Always let it warm to ambient temperature before you use it though.