Ok, here's the key. The tomatoes you buy from the grocery store are harvested before they are fully ripe. This is because they ripen on the truck on the way to your market. The problem with tomatoes harvested this way is that they lack the full sugar and flavor that can develop when tomatoes are vine-ripened until they are fully ripe. If commercial growers picked tomatoes when they were ripe, by the time they got to you they'd be rotten since usually tomatoes come from Mexico and so it takes weeks to transport them.
The home gardener can wait until the last minute when the tomato is fully ripe and at full flavor since they are consuming it within days of harvesting, not weeks like in the above scenario.
Protip: grow heirlooms, which are unmodified "classic" tomatoes with very complex flavors. My favorite heirlooms are Cherokee Purple and Black Krim. Amazing flavor, especially if you're letting them ripen fully on the vine. You know it's fully ripe when there is no green color left, and when the tomato itself is slightly squishy when you squeeze it.
Any other tomato questions?!