I am wondering how important it is to have an employer's reference letter made by your supervisor if he already agreed to serve as a reference person (for your resume/CV)?
The circumstances are that I quit working as a R&D intern (software/algorithm development) in order to proceed as PhD in computer science.
Is it common practice to have reference letters from your past employers or are reference persons in your CV state-of-the-art and sufficient?
Does this similarly apply to long-term company interns?
I ask this because I got the opinion that tailored recommendation letters (for a specific application) or contacts of reference persons are mainly used/asked in contrast to employment reference letter.
Sidenote: I do not have such a reference letter yet as I primarily studied until now and I do not want to regret having not asked now when there is the chance.
EDIT: It relates mainly to central europe (Germany) and research and development (close to academia) companies.