Trainers are Boring and Not Always Practical
Face it, putting your bike on a trainer and riding in your basement or living room can be about as exciting as a root canal. Sure, you can put on your favorite music or watch a movie and just "veg out," but doing this three to four times a week gets old fast. You will soon hit your "ennui threshold" and as a result, your adherence to your training program may soon plummet. Save these days on your trainer for one or two days a week when you need a long Zone 1 or 2 ride and have an engaging movie to watch.
There is definitely something to be said for the camaraderie in a group fitness class. You can get to know the instructor, and even share your training goals with him or her. If you're lucky and this instructor acts as a coach and not just a drill sergeant barking out orders, then he or she will probably take an active interest in your goals and help you achieve them. Classes are a great place for friends to gather to train together. Friendships can be forged that turn into outdoor riding buddies once the warmer cycling season arrives.
Music and Energy
One of the most common reasons given for attending an indoor cycling class is the music and energy of the class. It is just plain fun! Music plays a huge role in cycling classes, and can set the tone of the workout, can help set your tempo and can even transport you to another place through its emotional quality. Don't like the music your instructor plays? Try a different instructor or bring her a cd of your favorites and see if she'll add a few to her playlist.
A good instructor will provide motivation to help riders go beyond their self-perceived limitations. On days where you are meant to go hard, having a coach motivating you will help you achieve your desired intensity levels for those intervals or that threshold ride. Alone on your trainer, it's just not as easy to motivate yourself to go hard.
Most classes are 50-60 minutes. In that time you can get a phenomenal workout, often more than you can push yourself alone on your trainer. This is why it's better to leave the Zone 2 workouts for your trainer, and your higher intensity sessions for a class. That being said, you can always ride at an easier pace than the instructor is asking for if your own workout plan calls for a lower intensity ride.
Form and Technique
Indoor cycling classes are a great place to work on your relaxed riding form. If you do still ride outdoors in the winter, you may be focused on traffic, the road surface, riding in a paceline, or paying attention to your riding buddies, and you may not give your technique enough mental focus.
Indoors, you can spend as much time as possible committing a relaxed upper body to your subconscious. If you have a tendency to let your shoulders ride up towards your ears, indoors is where you can fix it. If your knees tend to fall to the outside as you ride, focus on changing that indoors. You'll see that pedal stroke skills can be enhanced to a certain point as well. I'll explain later the differences in the drive-train between an indoor and an outdoor bike and how it potentially impacts the development of your pedal stroke.
Through the camaraderie, energy, and motivation described above, adherence to a training program will be increased. Anything that will improve your adherence is worth doing, isn't it? Having an "appointment" with a specified class on a specific day will greatly increase your chances of doing that activity. It's so much easier to get distracted and put off a workout on your trainer at home. If you are coming home from work and plan to ride on your trainer at the same time as your kids are returning from school, dinner needs to be prepared, chores need to be done, the phone rings off the hook, and mayhem breaks out, you know the first thing to go is your own workout.
With your family squarely behind you, and with a fair trade for absenting yourself two early mornings or evenings a week, your indoor cycling class can become just like an appointment you must keep in your office. Studies have shown that when you treat your scheduled classes like any appointment at work and you schedule it and plan your day around it, your adherence to an event will greatly increase.
Part 2 will address the tendency of many indoor cycling classes to have little resemblance to anything you do on a bicycle and be more like "aerobics-on-a-bike". I will give you suggestions for how to seek out the best instructors with knowledge of cycling, and which popular movements or technique should be avoided at all costs.
In a nutshell, if you don't do it on your bicycle outside, don't do it on a bicycle indoors.