Making a kit wine is less labor-intensive than making wine from fresh grapes. So it's also much cheaper. You'll gain savings in time and money because you won't need to buy (or rent) the destemmers, crushes and presses that are required when starting with fresh grapes. To make a five-gallon batch of wine, you need almost 90 pounds of grapes, which could cost as little as $100 or as much as $400. Kits that yield the same volume run anywhere from $40 to $100.
Another bonus: Many kits are all-inclusive. They contain all the additives you'll need, pre-measured. The recipes are easy to follow and the results are fairly predictable. Recipe options allow you to add more concentrate for a bigger, grander wine. You can also choose to add less water than a concentrate recipe suggests. These two tactics would be akin to "letting the vats bleed" at a larger winery.
If you're a novice winemaker, a kit is a great way to start learning the art. If you're an expert fresh-grape winemaker, you should supplement your portfolio with a kit wine. It'll broaden your skills and deepen your knowledge.