Why should you consider writing a "new" resume & cover letter for each job you're applying to?

Because it could change over the years and don't need to reuse the old resume and cover letters

Suppose you created an amazing resume highliting all of your strengths that make you suitable for the job, and you got the job but later decided that the job was not right for you and you quit. not only did you have the opportunities to change and develop new skills but you could list your previous job as a reason they should hire you (depending on the amount of time you worked there). rewriting your resume might also give you practice to report what you have done, to your porental employer. the company may also keep public records of the resumes sent. If another potential employer sees your resume and realises that it is the same as the one at this other place they might realise that you did not put forth enough effort to create another one for them. then they might assume that you would not put your full effort forth while working for them. you might also want to Taylor your revealed skills to the business to optimise your chances of getting hired by them.