The shoes we run in today are great for reducing impact. There are a myriad of devices, gels, air channels, etc., designed to lesson the compressive loads on the joints and body. One drawback, however, is that if you do most of your running on even, paved surfaces the lower leg muscles and joints do not have to work as hard to stabilize. Joint stability and strength is crucial for injury prevention. This is where trail running comes in.
Running over uneven and varied surfaces makes the muscles of the lower leg work especially hard. Think sport specific strength training for the lower leg muscles. If you are new to trail running you may be surprised how sore or fatigued the muscles of lower legs are. The tendons, ligaments, and muscles all get stronger in response to this type of stress. Soft surface running also builds power and strength at toe off.
There are a few considerations when integrating trail running into your training plan. Unless you run in off road events the best time to incorporate trail running into your plan is in base or off season. As with any new type of training stress it is important to implement it gradually. You may want to start off with one work out per week of limited mileage and steadily increase the duration. Off road surfaces vary from crushed gravel, sand, grass, single track hiking trails, to rough back country trails. The more varied the terrain the more your lower legs and body will be stressed. On extremely rough and elevated terrain, hiking may be just as effective (and safer) than running. You will likely enter your aerobic base zones in this type of terrain as well, and may not need to run.
Trail shoes offer more support and traction but much less cushioning. Trail shoes vary from running shoes with a more aggressive tread all the way up to hiking shoes which may not applicable for running. Make sure you consult with a salesperson to get the right shoe for your type of training. I do not recommend doing any road running in a trail shoe, but you can take your running shoes off road if the surface is relatively stable, such as crushed gravel.
Trail running does not just work the lower legs, but all the muscles associated with running, including lateral knee stability. It also will help develop neuromuscular coordination. You can continue to incorporate trail running throughout the season for strength maintenance. An added bonus is that it adds variety to your training, and the scenery is better.