"Provisional patents" - more accurately referred to as provisional patent applications - can potentially establish a filing date with the USPTO which will serve as an assumed "date of invention" (unless this becomes disputed in court) in the event a non provisional patent application is filed relating to the same invention within twelve months. However, a number of inventor-filed provisional applications actually do not really end up offering any benefits at all other than allowing for use of the phrase "patent pending."
Provisional patent applications are easier to write for a layperson than non-provisional applications because they are not required to contain any claims (claim drafting requires a decent degree of technical expertise). However, claims aside, a provisional application must meet all the same technical requirements as any patent application. In short, there must be reason to believe that "one skilled in the art" would be capable of practicing the invention based solely on having read the patent application.
If this requirement is not met, a provisional application will not hold up in court as valid, nor will it even serve as proof of invention. Note that provisional applications are not examined by the USPTO.
This is NOT to say that a provisional application is never a good investment. They only run about $100 in fees to the USPTO, the phrase "patent pending" can be useful towards increasing credibility and discouraging knock-offs, and if you do feel that you can write a good application it never hurts to try.- Bill