This past spring I attended a seed swap. I didn't have much to offer this time, since it was my first time attending. However, I think I'll have plenty of seeds saved from the things I've grown so far. A few things I planted just for the seeds. Why? Because I didn't have enough for a full plot.
Saving seeds is a good way to ensure the survival of heritage plants. Some of these plants would be lost forever otherwise. They are generally hardy plants with good production levels. The haven't been genetically modified and the seeds grow true to the mother plant. They are open pollinated, so it's not hard to get viable seeds from them.
I did learn that for tomatoes and cucumbers, you have to put the whole fruit in a bucket or jar and just let it rot. Once rotted completely, you rinse them to get the seeds and let the seeds dry. This method is used to get rid of the gelatinous coating found on their seeds.