When visual design changes are made - it's exceedingly difficut to validate design changes automatically, especially when they don't impact the functionality.
Usability - if usability is important (it always is), an automated test won't tell you if you've made some changes that hurt it (like creating a div that hides buttons or links - the machine tests are not going to realize this)
Having people with otherwise free time, or who are between tasks or blocked waiting for something else to do their "regular" job perform tests. Time boxing the tests, as well as training the testers to look for specific things that we know the automated tests won't cover (like layout). Also, not giving testers complete scripts can maximize their effectiveness - meaning that they are more likely to exercise the system in ways we have not thought of, which will more effectively cover the application and turn up previously unknown issues at a higher rate than following a script.