I think I will skip over describing how object nav is actually used, as I'm not the best at making that part make sense. However, as an example I find it very helpful specifically in the following two programs.
The Windows settings program. Let's say you go to update and security, and then Windows Update. From there, you could use NVDA+B to hear if there is any update available for your machine, but depending on your screenreader's speed that could take a while. Alternatively, you could use object nav to navigate that area instead. Coincidentally, I don't think you can actually get to the check for updates button with just the tab key anyway.
This also lets you see more useful info in an easier to understand format, while you don't have countless other things being thrown in your face as well.
Another app as I indicated, which it is useful in... as much as I hate to admit it, is the HP Support Assistant program. Object Nav has been a necessity for use in that program. Otherwise, you miss a lot of information.
Another area I thought of just now is if you're transfering files from one place to another via cut/copy and paste. You can use object nav to monitor the progress of the transfer rather than using NVDA+B incessantly.
Hope this helps.