They may sound very similar, but both the public relations tools are different and solve a purpose.
Press Conference is a formal gathering that a brand organises at one set venue usually a hotel/ banquet to formally launch/ announce something for the media and consumers. The setup is such that easily facilitates one to one communication among important stakeholders and also gives the opportunity to showcase products live and have an on-ground team to explain the benefits/ features depending on the nature of the product.
On the other hand, a press release is a great tool to do a mass announcement (multi city) on the same day that can contain quotes from spokespeople. Press Releases allow brands to disclose information in a controlled manner about something that is about to happen or has happened in the passed as well.