There may have been numerous occasions throughout your time working for a company where you have achieved something you were proud of and highly valued by your employer.
One achievement may be that you brought on a new major client, or it may be a change in policy/procedure which your developed and implemented that resulted in a noticeable improvement in efficiency at work. Whatever the achievement may be, it is something that you would want to remember and record.
When you apply for another job, you can add those achievements to your resume to demonstrate clearly to the employer that you performed not only the required duties of your position but also that step further and attained several achievements while performing your role.
A potential employer may feel that if you have achieved so much in your last role, you may also do the same for their company and deliver more than expected.