Most files are designed to be run by a specific program. Including a file extension allows your computer to quickly identify the required program and launch it when you request to open the file. If the computer does not recognize the file extension, it may prompt you with a list of programs it has determined are most likely to open the file. If you know the correct program, you can select it from that prompted list, or search your computer for it. If the file does not contain a file extension, your computer will have no way of associating that file with a specific program.