The simplest answer is because they taste better, significantly so in fact. If you've never eaten a ripe tomato fresh from the garden, you may remain content to eat those pale, tasteless creatures the grocery stores pass off as tomatoes. But, if you've taken that bold step and eaten a fresh grown tomato, ripened on the plant and not in long term storage on some truck, I can almost guarantee that you'll never want to eat anything but fresh in the future.
The tomato pictured is a German Johnson variety tomato, just beginning to ripen (he got a slow start). This is an open-pollinated variety and is growing quite nicely without any special treatment in my main raised bed. The soil is just a mix of plain topsoil and well-aged compost from my own compost pile - there is no chemical fertilizer of any type added. As a non-hybrid variety, any seeds saved from this plant will breed true again next season, producing lots more delightful tomatoes.
Having my own tomatoes was the first and foremost reason I started gardening my little plot of land several years ago, as they have been one of my favorite veggies all my life and I use a lot of them for our meals here. In addition to the german Johnson, I have lots of Old Virginia plants, several Amish Paste, a Brandywine, and several dozen grape tomato plants that showed up as a surprise just outside of the bed, probably through the compost left over from last year. One way or another, I'll have lots of tomatoes this year, all from open-pollinated varieties.
Besides the superior of home-grown, vine-ripened tomatoes, there are two other factors to consider - cost and safety. The cost savings of raising my own veggies is easy to see, especially if I take the time to save seed each year to use the following year. These days, though, a bigger factor for many is the safety factor of home-grown versus store tomatoes. I like not having to worry about what weird chemicals have been added to a field or what unsanitary conditions might exist in that field. I know what my veggies are growin in, can the stores say the same?
It's really that simple - tastier, cheaper, and healthier. Grow your own.