why summarizng is necessary?

Answer 1

Summarizing any write-up or report at the end is preferred, recommended and appreciated for many reasons:

The whole of the write-up/report deals with many aspects in reasonable details. the writer's conclusion may be depicted at various places. To sum up, all such conclusions are put together for readers' convenience and consideration.

Similarly, facts might be analyzed one by one in paras but the combined conclusion or report is summed-up at the end for better understanding.

The whole write-up/report is never read repeatedly. The summary might be referred to time and again for consideration and the contents of the report gets recalled when summary is referred to.

The summed-up conclusion of the author/analyst/reporter may be reproduced and utilized for further references to take decisions and draw references in future.

Symmetrization helps the reader to refresh the writer's thoughts in his own mind after having read a long write-up/report and possibly dropping a few points due to human limitations.

Answer 2

Summarizing involves putting the main idea(s) or one or several writers into your own words, including only the main point(s). Once again, it is necessary to attribute summarized ideas to the original source. Summarized ideas are not necessarily presented in the same order as in the original source. Summaries are significantly shorter than the original and take a broad overview of the source material.
Summarize when:

You want to establish background or offer an overview of a topic
You want to describe common knowledge (from several sources) about a topic
You want to determine the main ideas of a single source