Hot-packing is the preferred way of canning foods. The less popular way of canning is the so called raw-packing. Reading the next paragraphs can give you better understanding of the two packing methods.
The popular hot-packing method is done by pre-heating fruits before filling into containers, then adding hot syrup or brine. The cap is half-sealed for processing in an open kettle then sealed immediately after. When processing with a pressure cooker, the container is fully sealed before processing. Time and temperature of heat treatment depend on type of fruits or vegetables.
Tin can is suitable only for pressure canner process.
Between the two methods, raw packing has a great disadvantage. Fresh foods contain about 10-30 percent air, it may include oxygen that cause oxidation.
Pre-heating of foods allows the removal of entrapped air in fruits and vegetables. Less oxygen inside the food container means longer shelf life. Air removal will prevent growth of most microorganisms except for anaerobic bacteria and will retain food color and flavors.
When I was in college, I had a different understanding about the two packing methods. I learned the hot-packed and the cold-packed instead. Hot-packed, The food is heated to desired temperature, packed and sealed immediately then cooled in a water bath. Cold-packed, the food is pre-heated, packed in bottles, half-sealed, pasteurized for a given time and temperature, jars are fully sealed and cooled in a water bath.