Built into Windows 10 and 11, Microsoft Narrator is an advanced screen reader with some useful features for sighted users and web developers. It visually highlights in real-time exactly what words are being spoken which could help users with low vision to both see and hear the content. Other screen readers, like NVDA, don’t do that.
When developer mode is activated (toggled on and off with CapsLock + Ctrl + F12) it masks off all other content on the page and focuses on what is being read aloud. This helps a developer to experience more of what it would be like to navigate the page as a blind user. A visually impaired person may not be able to notice changes made by AJAX methods in other parts of a page that a sighted person with peripheral vision could notice. If a developer is using AJAX to change something somewhere else on the page a screen reader user should be notified with an ARIA alert. In MS Narrator developer mode, the ARIA alert would appear near the area of focus. Then the user could choose to navigate back to the area of the page that changed. In some cases, a developer might want to program the focus to move the area that has changed. It would depend on what task was trying to be accomplished on the page. When moving the focus it would be a best practice to notify the user that they are moving to another part of the page to avoid disorientation. The philosophy of ARIA alert status update messages is covered in the WCAG SC 4.1.3. The intent of that WCAG success criterion is to make users aware of important changes in the page that are not given focus but do it in a way that doesn't interrupt their work or disorient them.
So, a good benefit to developers using MS Narrator developer mode is that it helps them see exactly where the focus is on a page and how shifting the focus unexpectedly could be disorienting to screen reader users. This might help them to program better alerts and notifications to inform the user what is happening.
Here is more information about MS Narrator Keyboard Shortcuts from Deque Systems. They also have a Narrator keyboard shortcuts single-page printer-friendly PDF version that can be useful for a developer to print out as and use as a cheat-sheet.