Provisional patent applications have value but its not always what the people who write them think it is. A provisional patent gives the applicant one additional year to file a non-provisional patent application but its no substitute for a real patent application. If you don't file a non-provisional within one year the provisional expires and you loose the one year extension that it brought you.
The most important value of a provisional application is to protect your patent rights from being lost due to public disclosure or offering for sale. If you do either of those things without filing a patent application, you start a one year period during which you MUST file the patent application or loose all rights to your invention which becomes part of the public domain. You loose all European rights on offering for sale or public disclosure , immediately. There is no grace period for European applications.
A provisional patent is filed but not examined. That means no one will look at your provisional application. It will simply sit on the shelf until such time as a non-provisional claiming it is filed. Then it is removed from the shelf and compared to the non-provisional. Any material which is in both applications gets the priority date of the provisional.
So what should be included in a Provisional application? Many attorneys don't include claims in a provisional or include just 1. The body of the provisional should contain everything you want included in the non-provisional patent. If you do this correctly, most of the information you want included in the non-provisional will get the "priority date" of the provisional.
Another useful property of the provisional patent is that it is not published. That means its not disclosed so if you so choose you can resubmit the provisional after it expires. What you have lost by doing this is the priority date, so anyone who happened to file on something similar to your invention during the year your provisional was in force, is now prior art which can be used against you.
For this reason we recommend anyone contemplating this strategy do a patent search for patents filed in the patent classes into which your invention falls , for the year that has just been lost due to the provisional's expiration.