In a must, we get the tannins, phenols, and color usually doing a cold soak for a few days, all depending on the varietal, ripeness, and acid. After that, we warm it up and pitch the yeast.
In a kit, the manufacturers have a process whereby they can extract color and phenols with heat as they make a must, then evaporate the excess water using centrifugal force and heat. After they have this concentrate, they mix it with juice, balance the acids, add extra oenological tannin that has been extracted from grapes and seeds, and call it a kit. All you need to do is add water.
The difference can be huge, but there are a few things that come to mind right up front.
Grape wines take time. Period. They also take testing, extra steps and organisms (like MLF), equipment, crushers/destemmers, presses, barrels, etc. After all that, if you know what you are doing, or even just lucky, you can end up with a fantastic wine! Or you can end up with something resembling vinegar you wouldn't even want to rub in an enemy's wound.
Now with kits, a lot of that work is done for you, and if you practice good wine practices and cleanliness, you will always end up with a consistent product that closely resembles their commercial counterparts. Not that kits typically have lower alcohol (12 - 13 %), and much lower tannins. This makes them age easy and makes a very drinkable wine within a year. Heck, they are even almost drinkable after 2 months.
Grape wines, on the other hand, take a very long time to even start to taste good. Wines under 2 years, done the traditional way have high tannins, high acid, that make them taste like crap! However, after a few years in tanks and barrels, they start to mellow out, become concentrated, and start to taste like $100 wines!
There is a big difference, but both have very significant merrits.