All athletes need to train pre season, in season and off season. There are numerous benefits. Athlete's need to be strong regardless of the sport they're participating in. It's pretty simple, if you want to get strong you need to lift weights. If you want to stay strong you need to keep lifting weights. 

Strength will do amazing things for the body. It will help you move faster, jump higher, increase bone density, increase muscle mass, increase power etc...  A good strength training program should always  include the basic 6, squat, hinge, push, pull, lunge, carry.   If you're not doing all of these on a regular basis you're missing out on great results! 

The one mistake I see a lot of athletes make is not training in season. With proper guidance and planning from a certified strength coach, an in season program is just as, if not more important than pre season! The exercises would not change much, rather the sets and reps would. 

Training can be done before or after games/practices. In fact, most high school, college and pro teams lift right after a game. They do this so they simply get more overall recovery time. A game may be held on a Friday night, they lift right after the game and get the whole weekend to recover. They will stick to a Monday Wednesday Friday routine. MLB teams lift right after their games also. 

The bottom line is, if you lift all year long but then stop during your season (3 months or more), you would lose everything you've gained over the previous 9 months. 

There have been studies shown that in season training athletes actually have gotten faster and stronger while in season, not in the off season! 

A few tests that I plan to use are the vertical leap, 40 yard dash, bench press, squat and hang clean. Testing is done pre season, and post season. If you have any questions about any of this, please feel free to contact me directly. Thanks!