What's the purpose behind a provisional patent application?

A provisional patent application is effectively a date-holder to provide an inventive entity with the earliest possible filing priority date for a non-provisional or PCT patent application. Ideally, the provisional patent application should be as complete, accurate, and thorough as a non-provisional patent application so as to in fact be capable of serving as priority for the later-filed non-provisional or PCT application - if there is subject matter in the later-filed non-provisional or PCT application that was not present within the provisional application, that subject matter will not be accorded the filing date of the provisional application but only that of the non-provisional or PCT application. The USPTO filing fee for a provisional application is substantially less than that of a non-provisional or PCT application, however, a provisional application will, by itself, never lead to a patent because the provisional application is never examined. There are several purposes for filing a provisional application - 1) it provides you a patent pending status; 2) it provides you a one-year time frame to file your non-provisional or PCT application, within which you can arrange financing, determine what commercialization of the patented system or method might look like; and 3) it enables you to begin to obtain protection on a device in connection with which you may develop subsequent embodiments - you can file additional provisional applications for such subsequent embodiments, effectively adding the new embodiment(s) to the first/original/preceding embodiments - then when you want to file your non-provisional or PCT application, you can simply file your latest provisional application as the non-provisional or PCT application within one year from the earliest-filed provisional application and you are assured that all embodiments have in fact been included in the non-provisional or PCT 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.

Filing a provisional application instead of a utility application is often the best strategy for a startup because it delays cost. The provisional application will buy you one year to experiment with your invention. If you pivot and the stuff your described in the provisional application is no longer relevant, then just ignore the provisional. If after one year the invention is still relevant, then you can spend the extra money to pursue a patent. Micro entity filing fee for a provisional is $65 Vs. $400 for a regular utility application. The main cost difference is in writing the application. A regular utility application can cost $6k-$10k Vs. $2k-$4k for provisional.