Yes, I am agree with all, What is robots.txt and its use.
"I don't understand what is the sole purpose of the robot.txt"
So the reason is, to use robots.txt :
(1) Every Website have two ends: 1. Front End & 2. BackEnd
At the side of Front End we can see our website which is exist the offering/services.
But at the side of BackEnd - User can change/Edit, Modify or can delete the website code, Web Pages, Users or functionality.
All the activities having their own url some are optimized (mainly all the front end part) and some not.
When we want to promote a website at that time we don't want to show those things which is not useful from users site
Ex. abc.com/admin - This is the admin entrance url, And no one want to promote this page even don't want those types of page will appear in Search Engine (Google)
So for the above reason we block the admin part by robots.txt
(2) Some time you want to change your web page or services, at that time you have to delete those old pages or have to redirect or if you keep as it is then robots those pages so search engine will never crawl those pages.