When applying for a job , why should the applicant use the proper structure an elements of a formal letter ?

Because it makes it easier for the person who is interested in hiring you to read, and you'll more likely be hired with a neat understandable letter.

When applying for a job, the applicant should always use the proper structure and elements of a formal letter because 1, it will be easier for them to read. 2, it will show to them that you are serious and properly educated. 3, it gives a first impression of the applicant (and first impressions are everything!) and lastly, 4, it just shows that the applicant can adhere to rules and standards which plays a big role in the criteria for most jobs.