Diagnostic assessment looks backwards rather than forwards. It assesses what the learner already knows and/or the nature of difficulties that the learner might have, which, if undiagnosed, might limit their engagement in new learning.
There are two main reasons for using a diagnostic assessment process. The first is to collect information that allows a teacher to write programs that match the needs of the class student-body. The second is to prepare a remediation program for students who have gaps in their knowledge, or who have mis-learnt information.