Motivation letter for tenure track position
The purpose is rather to convince them why you would accecpt it and that you are the right candidate for exactly this position offered (research, teaching) and that you have the right mindset for tenure, a professorship with its possibilities and duties and that you will become a successful and happy professor at that place in the netherlands (especially if you were not born in the netherlands and are no native speaker).
Therefore I would outline and epxlain:
- why you apply for exactly this position (goals (though this should be described in detail in the research statement), chances and perspectives, interdisciplinary local collaborations with colleagues and industry you see, link to your previous work)
- what is your philosophy of teaching, research, team work (international corpus of students at the local university, furthering female researchers,...)
- do you plan to engage in administrative work at the university, improving the strategy of the university
- what do you like about this place, local culture
- (what is your marital-status (not mandatory), language skill)
The last point should be covered in your CV. But based on your CV no one can judge your motivation for exactly this position! So don't underestimate the importance of the motivation letter. The lenght of this letter depends a bit on the number of applicants and you should know this number roughly and what the attention span for the letters will be. Of course it should be longer than a recommendation letter.
A cover letter is obligatory and always added for applicaton to academic positions from PhD up to tenure. A motivation letter must show and deliver arguments why apart from other candidates with similar publication record, grades, qualification you are the best match for this position.