What's the purpose behind a provisional patent 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.