The recent cast on Assessment Centres indicates that they are used for two reasons, Hiring and Development.  Most of my experience is of them being used for a third reason, firing (well, layoff/redundancy).

Where a company is looking to reduce headcount in a particular area, but not eliminate it entirely, but they cannot do so based on performance (perhaps they feel that their performance management system wouldn't stand up to legal challenge) they may use assessment centres in the selection process. 

e.g. Where I work is currently going through a redundancy selection process using assessment centres as part of the selection criteria (they count for 40% of the score, the remainder is based on performance record, non-performance disciplinary record, attendance record and length of service).  The assessment centre is split across two days (I think management were hoping that the first session would show wide enough variance that they could dispense with the second half).  The first was verbal and numerical reasoning tests.  The second is a 45 minute interview against put job description (which is a problem for some of us as most of our work is covered by the line at the end which reads "And any other duties assigned by the line manager") then giving a 5-10 minute presentation which we are given the subject of at the end of the interview and 15 minutes to research and prepare.