A brief history of the Finder – The Eclectic Light Company:

Thus, each Finder window could only show the contents of a single folder, and that location couldn’t be changed within that window. Navigating from one folder to another was accomplished by opening windows. It wasn’t uncommon to end up with stacks of half a dozen or more, each displaying the contents of a different folder, and Steve Jobs once unjustly criticised this as turning the user into a window janitor.

It’s difficult to explain to people who have grown up in the era of the GUI how advanced the Finder felt when it first appeared. As it developed, it got more complex, but the basics of how it works have come to dominate the way almost every user interface functions.