how important are motivational letters when applying to unis?

im applying to a couple of research and applied sciences unis in the nl next month(bachelors)and im planning on starting my prep for the admission and i realised i have to write motivational letters for each of them and some people stated that they are the “deciding factor” if i get accepted or not?is it true?sorry if this question sounds silly,my homecountrys educational system is completely different (only one program is numerus fixus if that matters)