There are plenty of external resources that get loaded onto a page that aren't directly relevant to the functionality you're testing (e.g., Facebook widgets, Analytics, JavaScript snippets, etc.). And these external resources have the potential to negatively impact your test runs due to slow load times.
Selenium is built to mimic human action (e.g., clicking, typing, dragging, dropping, etc.). So how do you use it to test for broken images?
Odds are pretty good that your production application has some kind of performance monitoring in place (e.g., New Relic). This goes a long way towards identifying when something detrimental has been released into the wild.
With Selenium you have the luxury of cross browser testing. But what happens when you also need to test how your application behaves on a _slow_ connection?
There are times when you are testing a page (or numerous pages) and you want to verify that it responded correctly. A great way to handle this is by checking the HTTP Status Code that the browser received. However this functionality is not available in Selenium WebDriver.