Given an arbitrary window in a spatial file manager, it must be possible to determine with complete certainty which folder that window represents. Furthermore, it must not be possible to change that association.
Traditionally, when a folder is opened, the icon representing the folder changes—perhaps from an image showing a closed drawer to an opened one, perhaps the folder's icon turns into a silhouette filled with a pattern—and a new window is opened. Attempting to open that "already opened" folder will simply reveal the existing window. A new window will not be created because that would violate requirement number one listed above, attempting to reveal an already opened folder's contents using another window will either close the existing folder before opening it or refuse to reveal said contents. The change in the folder icon's state is meant to be a visual reminder of this behavior. It says, "This folder is already open." Similarly, while a document or application is open, their icons also represent this status and re-opening them will just reveal and bring them back to the front.
A common alternative to the spatial file manager is the navigational file manager or "browser-style" file manager. A window in such a system is a device through which the contents of many folders may be viewed. A browser-style window is not tied to any particular folder. The window's spatial state (size, position, etc.) stays the same as the contents of many different folders are viewed through it. More than one browser window may show the contents of a particular folder simultaneously.
Furthermore, identification based on spatial attributes is a very natural human ability, requiring little or no conscious thought. The ability to recognize and recall locations within the hierarchy based on the appearance and position of folder windows is the primary purpose of the spatial file manager. All of the "rules" and behaviors that define the spatial file manager are designed to ensure that the strengths of the visual/spatial recognition and recall abilities of the human brain are leveraged. The idea is that these abilities are more natural and require "less work" than other forms of recognition based on reading text, maintaining an awareness of "current working directory" (in a command-line environment, for example), relying on the memory of past actions, or any other non-spatial cues.
One more advantage is that it allows the user to keep things arranged a certain way from one session to the next, as with the "workspace" settings in many high-end software packages. For instance, the windows showing the contents of different folders for a complex project could be tiled onscreen in such a fashion that all of them could be seen at the same time. In a browser-style file manager, one would be forced to rearrange these same windows every time they were reopened.
Proponents claim that this confusion is partially a result of the non-spatial nature of the Windows 95 file manager. When the connection between the spatial state of a window is not unambiguously and irrevocably connected with a particular folder, it becomes impossible to reliably recognize a particular folder based on its spatial qualities. Spatial state often becomes misleading in a non-spatial file manager. Users may incorrectly identify a window based on visual cues that do not actually link it to any particular folder, but are instead properties of the browser-style window itself. In such an environment, each new window adds clutter without recognizable meaning. This leads to the often-cited preference for a single window through which any folder may be viewed: a browser. The proliferation and familiarity of web browsers has strengthened this preference.
Also, maintaining spatial familiarity can be difficult when the file system is accessed from a variety of applications and devices with differing display capabilities. Reproducing a single spatial arrangement on many different display devices is sometimes impractical. Per-device, per-user, or per-display spatial state is one possible solution, at the cost of an increased amount of state information that must be stored.