The benefits of canning fresh vegetables include preserving the produce at peak freshness and in a relatively simple method. Water bath canning is only appropriate for high acid foods with a pH of 4.6 or less.
Canning started becoming popular in the 19th century as a means for preserving food that otherwise wasn't available much past harvest. It is still relatively inexpensive, and it saves produce from going bad. People can preserve vegetables, fruits, soups, sauces and meats with canning.
There are two canning methods, the water bath method and pressure canning. Using a boiling water bath is only safe for foods high in acid. This list mostly consists of fruits, though rhubarb is on the list. Pickling vegetables also makes them high enough in acid content to make water bath canning safe.
Foods with a pH of 4.6 or less have enough acid in them to prevent the growth of spores of the botulism bacteria. Unfortunately boiling doesn't kill these spores, so foods with a pH higher than 4.6 allow the spores to grow. Consuming foods with the botulism bacteria results in sickness. Examples of fresh vegetables that cannot be canned with the water bath method include asparagus, beans, carrots, corn and peas. These require pressure canning.
Pickling adds vinegar to the mix, which makes the produce acidic enough to kill off the botulism spores.