"Dream lofty dreams, and as you dream, so shall you become.
Your Vision is the promise of what you shall one day be; your Ideal is the prophecy of
what you shall at last unveil."
Wise man has said that every things happens twice; First time in your thought; second time in the real World.
This means that nothing can happen until and unless it has happened in our thought. Dreams are the seed of thoughts, that grows in to reality.
Therefore James said "Cherish your visions", because this is what you will achieve in future.