Deleting unused modules and deprecated versions of modules
Installation and Configuration
No real reason that comes to mind other than peace of mind during the update process.
When the update process happens the new module gets copied to its -version-2.2,2 location then the main symlink for the module pointer gets changed to the new directory. From that point the old directory is not used anymore.
If you found something was broken or missing from the new version and didn't want to wait around in that state until a fix hits the marketplace keeping a version gives you an option.