At school we were "forced" to summarize the text, but does it help in learning a new language?
Yes, it is useful in multiple ways:
Writing (or Speaking, if the task is oral) -it teaches you to express your thoughts in the target language.
Reading - it makes you read more carefully to comprehend more thoroughly.
Vocabulary - it is another exposure to the language. You will have to struggle to bring words into your mind while writing or speaking, which is a good thing by itself. We retain in our memory the words that we use better.