1.) The scenery gets beautiful really quick. Why focus on the road when you can relax in a Limousine and focus on your surroundings.
2.) Getting from vineyard to vineyard in a timely fashion can get complicated if you do not know the area very well. There are over 450 Wineries in Napa Valley and some of the best are pretty far from each other. Don't waste time getting upset with your GPS. Let a Chauffeur who tours the area daily, figure it out for you.
3.) Parking and Lines in the heat! Need I say more? It can get very warm in Napa valley and finding parking can be a headache, especially when it is warm. In a limousine you can sit back and enjoy complimentary open bar, or simply have the Chauffeur drop you in front and come back when you ready for your next winery!
4.) You don't have to hold back as much. Most responsible adults will limit themselves extensively when navigating between vineyards and fine dining so they don't miss out on anything. If a driver is simply waiting for you to whistle, you can do whatever you please until your bored!
5.) SAFETY! The drive back to the San Francisco area can be very pleasant. It can also be long and dangerous if you have had a little too much wine (very common on a wine tour!) If you are in a limousine you can kick up your feet, take a nap, turn on the TV, or let the party continue to rage while you head toward the city!