To date, all of the spectral line surveys towards extragalactic sources have been carried out at low spatial resolution. The large beam sizes in most of these studies just gather the averaged emission over the central few hundred parsecs in the central regions of nearby galaxies. Almost two decades ago, the review paper by Mauersberger & Henkel (1993) suggested that we could naively expect that the chemical variations in the different molecular cloud complexes, affected by different physical and chemical proccesses, would cancel out when observing the averaged emission over large regions. However, based on a number of previous studies and the more than two dozen of extragalactic molecular detections, they claimed that "surprisingly, the variations in the chemical composition do not cancel out" (Mauersberger & Henkel 1993). Their comparison of the molecular abundances of just seven species in the starburst galaxy prototypes, M 82 and NGC 253, where only three species were detected in both galaxies, showed that significant differences could be identified. Such abundance differences were attributed to the different heating mechanisms driving the chemistry in their central regions. It became then clear the potential of chemical composition comparative studies to probe the physical processes in the heavily obscured central regions of galaxies. Spectral line surveys, particularly at mm and submm wavelenghts, became key to understand and stablish the chemical classification and evolution of galactic nuclei.