Day parting is an important targeting technique that allows you to change your bid profile by time of day and day of week (which is technically called week parting). From a direct marketing/conversion perspective - this is important for several reasons:
1) Business to Consumer products/services have a different conversion profile than Business to Business products/services. This means that people tend to actually BUY at different times of the day depending on whether they are shopping for themselves or shopping for their work.
2) People often times research items at work, but purchase when they are at home - especially true for personal items.
3) The conversion on Business items drops dramatically on the weekends. If you are primarily concerned with conversions (sales) then the weekends are normally tough for B2B terms and products.

The more sophisticated PPC management platforms like SearchRev, eFrontier, Did-It, etc. have day and week parting built in as part of their bid profile management capabilities. However, these systems are rather expensive and require a large media spend to make the efficiencies worth the cost. Google has long had day and week parting as part of their free PPC ad management solution and we use it effectively for our B2B and B2C clients.

I’m looking forward to testing it with Yahoo’s new targeting capabilities.