Train the trainer is about providing trainers with the tools and material to run a professional training program internally. It is great for;
Cost effectiveness long term where there are more than one traunch of delegates to train. For example, Induction programs. You don't need to bring external trainers in to deliver an induction that is regular if you have internal resouce who are paid to do that job.
It's a great way to find potential trainers withint an organisation or even to utilise strengths within a company for a shorter program. For example a new product launch could be run in multiple sites through internal champions who provided with the training to ensure the message is delivered in a consistant fashion.
OR it could be used to develop a new or exisiting internal trainer who wishes to grow with the business they work for. They may want to learn new training techniques such delivery or design approaches.
However I would not advise you use a train the trainer program for your sales managers. Whenever asked to run train the trainer programs for managers I always ask;
"Do you want better sales managers or managers who are also trainers?"
The answer is usually what i expect "better sales managers who can train"
Hmmm?! Nice to have your cake and eat it but what I know they mean is they want their sales managers to stop doing and start coaching and developing. This is a management skill. Hence we train managers to be better managers. That way we can measure their skills through the sales activities and results.