The drooping of the binding energy curve at high mass numbers tells us that nucleons are more tightly bound when they are assembled into two middle-mass nuclei rather than into a single high-mass nucleus. In other words, energy can be released in the nuclear fission.
The drooping of the binding energy curve at low mass numbers tells us that energy will be released if two nuclei of small mass numbers combine to form a single middle-mass nucleus. This is nuclear fusion.