This article is from the Computer Security Evaluation FAQ, by Trusted Product Evaluation Program TPEP@dockmaster.ncsc.mil.
An evaluated product has the benefit of providing an
independent assessment that the product meets the criteria for
the rating it achieved. When considering a specific
installation the value of the data and the threat to that data
both need to be considered. These are often related, in that
more valuable data has a higher threat. If some of the threats
to the data can be countered by the features or assurance of a
trusted product, then it is certainly worthwhile to consider
that in your purchase decision. All other things being equal
(which is rarely the case) the independent assessment of an
evaluated product adds value.