|This is not a good system|
I am basically a lazy person. That manifests itself in a number of ways. One of those ways is a deep hatred of doing something twice. It burns me up. A subset of that is that I do not like losing information I have learned.
So I am rather meticulous about filing. First I had a filing system for my Bible studies and other stuff that fills two four drawer file cabinets. Since I have been on a computer for the past 29 years that filing system had migrated to my computer and is backed up continually.
This relates to the summary in this way. Summarizing your study of a book on one sheet of paper forces you to think through your study of that book at a different level. It forces you to record what was really important about that book for your life and ministry. It becomes a tool you can use to share with others.
For instance as you are talking through an issue with someone, you will remember that you saw that in that book. The summary helps drive that into your memory.
Application is the other thing that will do that, but that is another post.