“The United States provides a one-year grace period in which to file an application [for a patent] after a public disclosure, etc. Thus, particularly where a disclosure such as a public demonstration (as in a trade show) or publication is imminent, it may be necessary to file an application to preserve patent rights.
Under either the absolute novelty or one-year grace regimes, patent applications are frequently filed with a certain haste, to avoid patentability being barred by a prior art disclosure. Thus, the perception that a provisional patent application can be filed quickly, informally and inexpensively…
The common perception that provisional applications represent a low-risk, quick and easy way to obtaining a priority date with minimum effort is misplaced. In fact, the false sense of security with which inventors proceed to publicly disclose their inventions after making provisional filing may ultimately render the invention unpatentable. ”
Read more in this McDermott Will & Emery article from Mondaq.