The SEAL ONLY tray packers are the simplest version of the tray packer models. The machine simply heat seals film onto trays leaving any atmospheric air inside the tray with the product. These tray packers are generally used in the industry when shelf-life is not as important.
Seal only tray packers are a quick and simple ways to put a film over the top of a tray to keep the product inside. The air inside the seal tray is not an inert gas so won't massively improve the shelf life of the product. Sometimes this is used by restaurants who have a central food processing site and then distribute sauces, products etc to their restaurants for them to finish off at the time of serving to a customer.