You don't have to justify why it is beneficial to the forces. I am going with the fact that if an employee is happy or content in their work, their productivity, lack of sick days etc speaks for itself. If you feel the need to justify or come up with a reason in the memo, you could put something like: After X years in the service/in my current trade, based on my interests and aptitude, I wish to continue my career in the CF in the X trade.
Or some such thing.
The military generally benefits because you take time in, rank, experience and training and keep it in the forces so to speak. Trade skillsets may not be compatable but the experience and rank etc does carry forth.
It is not too late to start this, I started my VOT in October last year. Check out the tips and tricks for VOT, they will help you manage your timeline.
Good luck!