Because there are typos in their thesis and passages that need editing.
That’s it! That’s all! Enough said!
OK a little more… 🙂
Writing a book is hard. Proofreading is hard. Combining these both in a project with a word count in the tens of thousands means the resulting thesis will likely have mistakes that need correcting.
Some thesis corrections are simple. A missed or misspelled word is obvious when spotted.
Some thesis corrections are subtle. They take patience to see and consideration to correct.
Some thesis corrections are style-choices. Examiners might feel something is needed and usually their requests are followed.
Remember that all thesis corrections are requested with the goal of making the thesis better. Most candidates get asked to complete corrections. Expect that you will too, get them done when you’re asked and then you’re done!