• Fast Quality Check - The most important reason to do daily builds is to be able to test the prior day's changes.  It is crucial to catch problems as early as possible, ideally the same day, rather than let them accumulate over days or weeks.  
  
• Ensures Buildable Check-ins - Another critical reason for daily builds is to ensure that the code repository is always buildable.  It is very easy for people to break the build, which tends to stop all other developers as soon as they sync their workspaces.
  
• Cross-platform checks - In multi-platform environments, developers typically do their work and local tests on one or a few platforms.  This can easily result in problems on other platforms, so doing a daily build on all environments will root out these issue quickly.  This is particularly important because some platform issues require component redesign to work properly and this should be done as early as possible.

Good automated regression tests are required for this to work properly, so building a automated test framework should be an early goal of the QA group, even before they have a product to test. 

Another facet of daily builds is to encourage frequent check-in, ideally daily.  This generally forces developers to make changes in small, digestible chunks that are easily understood and managed.  This isn't always possible if major changes are going on, but it should be the goal.