On a VHS, you can pause a video playback and go forward or backward frame-by-frame to inspect a scene that you might have missed because it was played too quickly.
Similarly, when browsing the Internet, a URL may change very quickly, but the user actually wanted to go back to inspect that certain URL. In frame-by-frame navigation mode, you can do that, because sudden changes in the URL, such as by JavaScript redirect, are intercepted and awaits for user confirmation with a unobtrusive dialog embedded at the top of the page before going to that new URL. Some examples:
1) When a browser tries to load a URL, call it "X", and the HTTP response header returns a redirect address, or there is an http equivalent redirect specified in the HTML meta tag, "X" is skipped when pressing the back button later. This is the standard browser behavior in order to make the back button actually function.
In frame-by-frame mode, "X" is not skipped. You can go back to "X", and any automatic URL changes are intercepted. The user can then inspect what the url address "X" was, or, if it was an HTML tag that specified the redirect, the user can take a glance at what content that HTML page showed, if any.
2) When loading a page with URL "X" that does JavaScript redirect, as opposed to an http response header redirect, this can cause a problem later when the user presses the back button, because the browser may faithfully load "X". And when "X" is loaded, the redirect is automatically triggered, making it difficult for the user to go back twice. Usually when this happens, the user has to press down on the back button to get the browser history menu and skip a URL from that.
In frame-by-frame mode, when the user goes back to page "X", the auto-redirect is intercepted, allowing the user to press back again to go before "X".
3) Page "X" embeds within its page, another page "Y" from another domain, using iframe tags. The content author of the page "Y" that loads in the iframe does not appreciate this, so page "Y" has a javascript redirect that causes page itself to break free from page "X". That's fine and dandy for the author of "Y" because it gives the author more control. But what about the user's control?
In frame-by-frame mode, the user gets to arbitrate between what the author of "X" wants and what the author of "Y" wants. By preventing the page from automatically changing its address without user control, the user can now press the back button and get to see "Y" in "X" as the author of "X" intended, so that the user can view what's in "X" and maybe click something useful on that page. (e.g. diggBar, Google Images)
Those are three examples, but there may be more unlisted cases where giving users the control over the content authors' is benefitial to the user, in the form of frame-by-frame navigation. However, this is a tradeoff between the conveniences of having everything automated for you versus being able to control everything. In this case, the control over URL changes.-- rhatta, Apr 29 2009random, halfbakery