With fewer people growing large gardens lately, and the coincidental advent of hybrid vegetable varieties, this practice has decreased markedly. Seed saved from open-pollinated vegetables can be expected to produce offspring similar to the parent plants, assuming that only similar plants are allowed to contribute pollen and make seed. The parent types are generally highly inbred, so they will express their genetic traits uniformly. Because of this, you generally cannot expect satisfactory results saving seed from hybrids. The resulting OP will not be as vigorous as the original hybrid, however.
Hybrids tend to be more vigorous than OPs, a well-documented phenomenon known as hybrid vigor .
A second benefit to commercial seed growers is the parent lines of a hybrid can be kept confidential. If they develop a superior OP, competitors can just buy some of the seed and grow it for themselves. If they develop a good hybrid, it can't easily be stolen. References on seed saving and storage Growing Garden Seeds, by Rob Johnston of Johnny's Selected Seeds. This is a good, inexpensive reference for anyone who wants to know the basics of seed saving and seed storage.