Your cover letter is one of the most important parts of your application. If you are applying direct to an employer it will be one of the first, if not the first, things the hiring manager reads about you.  It is therefore important to ensure that your cover letter portrays you in the best light and that it is accurate.  

A common mistake is using the same cover letter and simply editing sections to try and make it applicable for different jobs. Recruiters can usually tell when this is happening as the letter is usually not tailored to the role or doesn't hold any relevant information. I've even received cover letters that relate to working at a different company! 

It is important that the cover letter shows you are the best candidate for the job and therefore you should write one for each role that you apply to.