One possibility is that it can be deleted safely and may be out of sync with the project because of changes that the IDE could not understand.

To test this, you can create a backup folder outside the project folder, move the folders there, then Run the project again.

For example, the contents of a build folder (whether it's compiled bytecode or other artifacts) may be created each time a compiled language is compiled. This folder and its contents may be created by the IDE each time the user runs the program.