The quiet period allows the SEC to complete the review process without bias or interruption and ensures that the company doesn’t attempt to hype, manipulate, or pre-sell their stock.
Companies are allowed to discuss information already in the prospectus during the quiet period, and oftentimes they will go on a “road show” to present this information to big, institutional investors and get a sense for the potential market. Activities generally avoided during the quiet period are advertising campaigns, conferences, and press interviews—basically, anything that might generate public interest in a company or its securities.