Provisional patents have the advantage of being low cost but they have another additional advantage that make them a plus for inventors.

Requirements per the US Patent Office

The provisional application must be made in the name(s) of all of the inventor(s). It can be filed up to 12 months following the date of first sale, offer for sale, public use, or publication of the invention, whichever occurs first. (These pre-filing disclosures, although protected in the United States, may preclude patenting in foreign countries.)

A filing date will be accorded to a provisional application only when it contains:

• a written description of the invention, complying with all requirements of
  35 U.S.C. §112; and
• any drawings necessary to understand the invention, complying with
  35 U.S.C. §113.

The requirement of a provisional patent specification is:

The specification shall contain a written description of the invention, and of the manner and process of making and using it, in such full, clear, concise, and exact terms as to enable any person skilled in the art to which it pertains, or with which it is most nearly connected, to make and use the same, and shall set forth the best mode contemplated by the inventor or joint inventor of carrying out the invention.

No claims are required in a provisional patent which is an enormous advantage.  You can put into a provisional patent every possible variation of your idea, even though it might require more than one utility patent, and still have a one year priority period for all the concepts as long as the specification is detailed enough that a person skilled in the art could make and use your invention.  So you can put in many ideas.  And you don’t have to format your specification in a rigid way.  You could send in a copy of your entire inventor’s notebook and as long as a complete description is in the notebook, you could claim priority.

One of the main problems inventors have is they apply for a patent and then through market testing or actual use find out their idea needs to be changed, and that those changes are not covered by their existing patent.  First that costs money to do a second patent, but more importantly another person could see your idea and apply for patent of the modified idea before you do, which would not allow you to patent the changed idea.

So the provisional patent allows you to actually receive temporary patent protection in as many variations as possible, so that you don’t have to decide on your final patent configuration. When you apply for a provisional patent include a specification but also include every piece of research, tentative ideas, brochures and any other material you have. This could have a big payoff if you change your design approach to your product.