Q: How to write the rationale of a study?
How to write the need of the study?
The term used to imply why the study was needed in the first place is "rationale for research" or "rationale of a study." I have edited your question to reflect this.
The rationale of a study is a very important part of the manuscript. You can do a thorough literature review to understand what is already known in your field of study and thereby identify the gaps in knowledge or the problems that are yet to be solved. Such gaps will form the framework of your study and outline the need for a new study.
Thus, in the Introduction section, you will need to provide a background on the topic and what is known on the topic. The next paragraph would indicate the gaps in the current literature and then lead to the goal/objective/rationale for your new study.
Your question is unclear. Are you referring to the "Thesis summary" that is often a part of the first chapter of a thesis along with the introduction? Or do you want to know how to write the summary of the first chapter of your thesis?
If it's the latter, you would need generic inputs on how to write the summary of any chapter of a book or a thesis. Basically, a summary of any chapter would just highlight the main points that have been discussed in that chapter. It should be a highly condensed version of whatever you have covered in that chapter.
If you wish to know about the "Thesis Summary," I would suggest you first go through the guidelines provided by your univeristy/institution. Each university has different requirements and while some require a chapter-wise summary of the entire thesis, others focus on the main elements, such as problem statement, methodology, reults, conclusion and implication. You can also clarisy your doubts with your supervisor before you start writing.