What's the purpose behind a provisional patent application?

Another advantage of a provisional application is that should you decide that your invention disclosed in the provisional application is not ready to be commercialized, you can allow the application to lapse without the contents of the application being made public knowledge. This provides you the option of re-filing the patent application at a later date without the earlier provisional patent application being used as prior art against your re-filed application (do note, however, that the re-filed provisional application will have a later priority date). This is not possible with a standard patent application whose contents will be published some time after filing. Allowing a standard patent application to lapse often means losing your rights to the invention disclosed in the application.