Step 1: Read the question.
We must find the common use of achievement tests.
Step 2: What an achievement test is.
An achievement test measures what learners have actually learned after the teaching is done. Examples are school exams and unit tests.
Step 3: What it checks.
It checks the knowledge gained, the skills built, the understanding of concepts, and the overall progress of the learner.
Step 4: Its main purpose.
By looking at the results, teachers can see where each learner is strong and where they are weak, and then plan extra help.
Step 5: Check the other options.
Ranking, selecting for a course, or picking for a job are uses of other kinds of tests, not the main aim of an achievement test.
Step 6: Final answer.
Achievement tests are commonly used for identifying the strengths and weaknesses of learners.
\[ \boxed{\text{Identifying strengths and weakness of learners.}} \]