Why are measures of central tendency essential to descriptive statistics?
Because in describing a set of data, our main interest is usually the central value of the distribution.
In descriptive statistics, we are explaining the characteristics of a set of data in hand - we are not making conclusions on the larger population from where the data come (That's inferential statistics).
In doing so, our main question is usually 'where is the center of the distribution'. To answer that question, we normally employ either the mean, the median or the mode, depending on the type of data. These three central tendency measures indicate the central point around which all the data gather. That is why it is one of the two essential parts of descriptive statistics. The other part is the measure of dispersion, which explains how far data is distributed around the central tendency.
So with central tendency, we know the center of the distribution of data. With dispersion, we know how spread the data are.