Indoor cycling provides a lot of benefits that transfer to cycling outside.  For beginners, it is a good way to build up leg strength and the cardiovascular system before taking long rides outside.  In the controlled environment you can monitor both power output and heart rate to build a good base.  Once you are riding outside, you can use the indoor rides to help monitor progress or just use it as an easy recovery day by removing additional stresses such as wind, heat, cold, or steep inclines.  Mixing in other cardiovascular equipment such as ellipticals and treadmills can be helpful to prevent overuse injuries and to work other muscle groups.  Adding circuit training and weight training to the mix will also help with muscular development and injury prevention.