It seems to me that in most positions you should be able to get a decent feel for the technical abilities of a person based on the first hour or so of an interview, along with their resume, reference, and maybe a technical test. So what benefits does a company reap when it spends several man days worth of their employees time to keep them occupied all day long in an interview. The interview process is costly enough with out making it longer than needed.

So why is it that many companies have shifted to a day long(sometimes more) interview for professional positions? There must be some benefits that are consistently noticed for companies to make this shift. What are they?

NOTE:

I am looking for a great answer.
To me that great answer will many exhibit the following traits:

• Will be comprehensive in that it attempts to cover most of the general benefits realized by companies using a full day interview process
• Includes references to claims of benefits that have been put out by companies
• Includes references to research or statistics that have been published to back up claims.

I intend to offer a significant bounty for an answer that does a great job of answering this question.