In order to be able to answer all scientific inquiries into a universally intelligible format, one has to develop a commonly accepted language in which to converse. It was this need that led to the development of units and dimensions. It is an effort to do away with the subjectivity of forms and personal prejudices and introduce common objectivity.
From the IIT JEE point of view Unit and Dimensions, Vectors, and Scalars do not hold a lot of significance as we cannot expect a number of questions directly based on this. However, we cannot completely ignore this chapter as this forms the basis of all chapters to follow.