I agree with the point lmetcalf made about the length requirement forcing students to be clear and concise with their summarization details. Sometimes, however, this might be a handy strategy for the teacher to use in order to entice readers into an upcoming reading assignment. For example, students might be reluctant if you gave a lengthy description of Hamlet, but a short one that hits the highlights--love, lust, murder, betrayal--would certainly entice the readers.
When I read this topic I thought of the shrinking process as one that would be done by students as a method of reflection and reveiw of a novel or play. If I am required to sum up a book in 500 words I have a lot of room to play. If I have to sum up a book in 150 words, I have to really think about what the most essential ideas are and it forces me to think about everything I might want to include and then edit. It is a really challenging task! I would go so far as to say it requires a lot more thinking about the text -- hence it is a great review technique.
Synopses of classical works, like other sources, are subject to misuse. While they are certainly not meant as the be all/end all, they do help in the ways that the second post mentions. For, unfortunately, many students nowadays are not reading at the level at which they should. If a student reads a summary of the work, then he/she has some grasp of the plot. So, when the student reads the classic itself, hopefully, then he/she can pick up on the beauty of the language and the nuances of theme and other elements.
For the same reason that parents buy the children's version of the classics--that of exposing their child to some of the greatest narratives--synopses are written. We have them here at enotes, of course. But, they are provided as study aids only, not as alternatives to reading the play or book or story. The intricasies of a novel or story are missed without one's having actually read the work. But, a summary can assist in comprehension, to be sure.
Tough question. This one is going to be hotly contested on different sides of the issue. I am going to step aside from the "dumbing down" element and try to spin it in another way. I would say that one of the reasons why classics are abridged to brief synopses would be in the hope of getting more people involved in reading them. Certainly, they are not intended for those who have read the work. Rather, they are constructed for those who have not as a potential invite to further their understanding. After reading a synopsis, someone can decide that what has been read has interested them and it is worth continuing. I am fully cognizant that many abuse this and simply read the synopsis to evade reading the entire text. Yet, I do think that it is quite valuable in order to have a widened glimpse of something as almost a pre- reading activity in order to generate more interest to continue study within it. Along these lines, I think that students who are studying the classics can find the synopsis to be quite helpful in order to supplement their reading. I will fully concede that some (more than some?) do not adhere to this, and rather use the synopsis to supplant reading the entire text. Yet, in this concession, I will also point to the fact that there are more than a few teachers in the world who assign a mammoth of literary challenge without any resources whatsoever to help out their students. This happens on both secondary and collegiate levels. Students who are pitted in such challenging conditions, where no differentiation nor support for a landmark of literature is offered, have little other choice than to turn for a synopsis as some guidance. Think of the struggling student who has been assigned to read Shakespeare without any resources, any handouts, any prep work, and/ or any activation of prior knowledge. Where is that students supposed to turn but to a synopsis? The arguments against synopses of classical literature and literature that fits the capital "L"iterature are very persuasive. Yet, until all teachers recognize that assigning students challenging literature must be done with some level of assistance in both in class and out of class teaching, I don't know where else a student is supposed to go other than a synopsis.
I assume that you are asking why they do this in the book, rather than why we in the modern world do it in places like Cliffs Notes...
The classics have been shrunk to brief synopses in the history of this society because the people are losing their ability and their desire to concentrate on things for long periods of time. They are also losing their desire to think and to have nuanced and detailed works to think about.
The process of shrinking the classics is part of the general dumbing-down of the society. People stop really engaging with the works of literature -- they stop thinking about them. Instead, they just consume them in little bite sized pieces that don't take much time or effort. This is kind of a step on the way to ditching the classics (and all books) altogether and just getting entertainment from "parlour walls" and from trying to run people over in your car. ????
We live in a time when coffee has become 'instant' coffee and food has become 'fast' food. People just don't have the time or the inclination or patience to spend long hours over many days to read the original version of a timeless classic. Whatever gratification and satisfaction people wish to derive from reading has to be immediate. Hence the need for synopses of the great classics.
But, in itself it is not a negative trend if it makes the reader who has read the synopsis to attempt to read the original. Many readers after reading the short summary of a classic have gone on to read the original.
Many readers even as they are reading the original classic often refer to its synopsis in order not to lose their bearings. So, most certainly a synopsis has its usefulness and value as long as it does not become the substitute for the original.