It has to do with how your brain works and sorts things into short-term memory or long-term memory.
Information learned over time, and repeated at intervals, goes into long-term memory. Information "learned" once only in a short span of time goes into short-term memory.
The practice of "cramming" is the classic example of learning something only long enough to pass a test, and as soon as the test is done, the information is likely lost and forgotten.
Would you want your brain surgeon to have studied for his finals only by cramming?