The great thing about this subject these days, is that it is universally accepted that it is essential for all teams to partake in some kind of pre-season training.
I biggest mistake made is a misunderstanding of what pre-season training is. It's not sitting in a gym pumping weights for 3 months or running 40 laps of a football field.
Pre-season training for me comes under 3 headings.
1. Performance Enhancement
2. Injury Reduction
3. Reducing burnout and boredom
The evidence out there supporting how much a well designed pre-season program can enhance physical performance is huge. The only variable can be the well designed part. Putting a gym into a sports club or joining a gym is a great idea but the real game changer is the coaching and design of a program. Get all this right and pre-season will have a big influence on performance levels. Do all this over 2-3 years and you will really see the benefits.
I any good program there will be strength exercises to build strength and resilience throughout the body. There will also be mobility training integrated into to the program to build better movement and function into athletes. This is an amazingly effective way to both reduce injuries and also reduce the length of injuries if the do happen.
Reducing burnout out and boredom:
There's a limit to how much on field or on road or whatever your sport is you can do without it becoming repetitive both physically and mentally. A good pre-season breaks this up and leaves your athletes fresh and strong when it counts.