Focus is another thing that improves if a summary is properly utilized before reading a play. Since I wasn't preoccupied with remembering who is who while reading the beginning act of Love's Labour's Lost, I was able to pay more attention to things like rhyme (although that's really hard to miss anyhow) and the nuances of different characters' personalities.
Now for a more subjective reason: it is a necessity to better enjoy the the overall story. I know I don't speak for everyone on this, but knowing the end of a story usually doesn't hinder me from wanting to read it. In fact, in many cases it causes me a greater desire to see the process, or the story's development.
The math's not perfect, but a simple sum might be:
summary + text = greater reading comprehension