Why KSA in the Federal application process?

KSAs - Knowledge, Skills and Abilities - a list of special qualifications and personal attributes that you need to have for a particular job. These are the unique requirements that the hiring agency wants to find in the person selected to fill a particular job. 
​A primary purpose of KSAs is to measure those qualities that will set one candidate apart from the others. In federal personnel guidance, KSAs are defined as the factors that identify the better candidates from a group of persons basically qualified for a position.

How well an applicant can show that he or she matches the position’s defined KSAs determines whether that person will be seriously considered for the job.

Knowledge statements refer to an organized body of information usually of a factual or procedural nature which, if applied, makes adequate performance on the job possible. A body of information applied directly to the performance of a function.

Skill statements refer to the proficient manual, verbal or mental manipulation of data or things. Skills can be readily measured by a performance test where quantity and quality of performance are tested, usually within an established time limit. 

Ability statements refer to the power to perform an observable activity at the present time. This means that abilities have been evidenced through activities or behaviors that are similar to those required on the job. Abilities are different from aptitudes. Aptitudes are only the potential for performing the activity.

Why You Can’t Ignore Them?

Some job applicants, especially those looking for their first federal position, are not sure whether the KSA requirements apply to them or where a response is optional. 
Obviously the job applicant will want to focus the most effort on responding to the more heavily weighted KSAs or the mandatory ones, but it is important to remember that you need to address every one on the list. 
A key point to remember about all KSAs is that they must be job-related. An agency cannot ask for anything in a KSA that is not in the job’s position description.

The Writing Part

Do not assume that reviewers will pull the information out of your application package and pick up the salient points. They will not. Even if there were time for such insight, reviewers/ evaluators are not allowed to infer anything.

Hints to follow when developing your KSAs:

• Read the vacancy announcement carefully
• Gather the information needed to begin writing
• Be specific
• Be precise
• Get to the point
• Do not ramble
• Use lots of examples
• State specifically what you have done
• Do not use acronyms
• Present yourself in “clear and plain” language
• Do not borrow language from the position description

There’s an easy way to decide when to pay attention to KSAs. In a word, ALWAYS.

A few more details can be found in this article.