Provisional patent applications can typically be filed more quickly and less expensively than can a non-provisional application. If, however, you are certain that you will eventually proceed with a non-provisional application, filing a provisional and then a non-provisional is typically more expensive than just beginning with a non-provisional filing. Accordingly, provisional filings may make sense if you are unsure whether you will ultimately proceed with a non-provisional filing and where you wish to minimize your upfront investment.
Beware, however, of any attorney or agent who offers to file a dirt-cheap provisional patent application for you (for example, a provisional application for less than one thousand dollars). Though provisonal applications face few formal requirements, they only provide value as a priority filing if they satisfy the rigorous patentability standards of written description, enablement, best mode, etc. Explaining the details of these requirements would require too much space for this post, though, in short, the invention needs to be described in full and exacting detail. If your description and drawings are in any way incomplete (or even if you describe the invention in the wrong way), the provisional filing may have little or no value and your invention will not be meaningfully protected.
While any registered attorney or agent can file a cheap provisional and there are numerous scams that offer to help you file your own provisional application for a modest fee (see, for example, the complaints on the "scam prevention" page on the USPTO website), you might compare it with surgery. Anyone can cut you open and remove things from inside you, but would you trust anyone to do so other than a highly qualified surgeon? And for that professional service, you will likely need to go to a hospital and pay market rates. Moreover, in most cases, you would not try it yourself unless you completed medical school. Patent law is every bit as complex as modern medicine; using one errant word can be every bit as devastating in terms of forfeiting your invention as might some hack nicking your carotid artery forfeit your life; in either case, it may well be game over.
Those warnings aside, allow me to address some of the other benefits of filing a United States provisional patent application.
First, filing a provisional patent application allows you to market your invention (which is described in the provisional patent application) as "patent pending." This claim can serves as a powerful marketing tool, and seeing "patent pending" on your product will often give pause to competitors who may want to steal your idea. Because you have locked down a filing date with your provisional filing, you may also then be able to begin marketing your invention (assuming it is fully described in your provisional filing) with reduced risk of anyone stealing your idea.
Second, if you are looking for funding or investment for your technology, potential investors will generally love to see that you are pursuing patent protection; and often a provisional filing will be enough to satisfy them that you have taken adequate initial steps to protect your technology.
Third, provisional filings allow for up to an extra year of US patent term. An issued US utility patent is valid for 20 years from the earliest non-provisional filing date. Consequently, you can file a provisional patent application today and then wait one year before filing your non-provisional patent application. Your priority date will be today (if you fully describe your invention in the provisonal), though the clock on your 20-year patent term will not start ticking until November 2010. The "priority date" is important because it determines what can be asserted as "prior art" against your invention.
Fourth, provisionals can be very valuable when you are still engaged in R&D and your technology is still evolving. You can have a provisional application filed today to cover the core idea; and then, assuming you generate new ideas and improvements over the next year, those new ideas can be added into your subsequent non-provisional filing. In the non-provisional filing, your broadest claims my be supported by your provisional patent application, and thereby be entitled to the priority date of the provisional filing, while your non-provisional application can also include more specific claims directed to your later improvements that may not have the benefit of the original provisional filing date, though which may still be patentable.
Fifth, even with the best of intentions, emergencies happen where a disclosure of an invention may suddenly become imminent. A provisional filing can be made (often same-day) covering as much as possible within the time avialable. While there often will be risks that the disclosure in the provisional application will not be sufficient, at least you can preserve a chance to obtain patent protection; and a skilled patent attorney may be able to work wonders even with just a few hours.
Sixth, if you do not follow up with the filing of a non-provisional patent application, your provisional patent application will go abandoned and can remain secret forever. Consequently, you may be able to allow your provisional patent application to lapse and then rely on trade secrecy with no one ever learning of the details of your invention.
Any legal advice as to whether you should file a provisional patent application obviously is very fact- and context-specific; and you should talk with a qualified patent attorney to assess the right choice for your situation. Additionally, this post really only scratches the surface regarding provisional patent application, though I hope it offers some useful information.
Relatedly, if you wish to see typical costs for provisional and non-provisional patent applications and the various stages of patent examination and patent maintenance, you are welcome to obtain my free report on typical patent costs.