Why Computrainer in the Summer?
"Coach, why on earth would I sit on a CompuTrainer when it is so nice outside?"
As a triathlon coach, I find indoor trainer sessions during the race season very important. It is so much easier to control the variables of your training session, how hard and how long, without having to worry about traffic, lights, or road debris.
Training indoors also enables the rider to work at a specific cadence, heart rate, or power as there are no inclines or headwinds to contend with. The same applies for intervals. Athletes can perform the workout according to plan. Also, if a smooth, efficient pedaling stroke is an issue for you than an indoor trainer session would help build that muscle memory.
Indoor training is also great for the mental side of racing. Riding in place for an hour or more forces you to focus on the task at hand. There are no potholes, no cycling buddies, no welcome changes like a tailwind or winding descent. There's just you, your bike, and four walls. Pedaling on a trainer obliges you to concentrate on your breathing patterns, your pedal stroke, and—you guessed it—your pain. This makes indoor cycling the perfect way of testing sport psychology techniques.
Indoor training also allows you to train in any weather and at any time of day, so if you are a time crunched athlete, indoor training may be your best option.