The purpose of diagnostic testing is

The diagnostic test refers to the test which helps the teachers to know the learning difficulties or gaps in the learner's understanding and identify the learner's strengths, weaknesses, skills, etc.

Important Points 

Steps of Diagnostic Testing:

• Identifying students who are having trouble.
• Locating the errors and learning difficulties/gaps

• Discovering the causal factors of slow learning.

Key Points​ 

The Purpose of Diagnostic Testing in Mathematics:

• This test helps to know the gaps in children’s understanding.

• It is specially conducted for removing the learning difficulties of learners. 
• The diagnostic test enables the educator and students to recognize issues that they have with the subject. It helps in improving the teaching and learning process in mathematics subjects.
• It is a comprehensive test that provides feedback to teachers and students on their strengths and weaknesses. 
• This test provides a way to identify the part which makes the child slow down in the learning process and provide appropriate feedback to the child.

Hence, from the above-mentioned points, it becomes clear that the purpose of diagnostic testing is to improve the teaching and learning process in mathematics subject.