Nothing.
First, let us compare Darwinism against real scientific theories like electro-magnetism. If electro magnetism is true, then we already know what there is to gain by electro-magnetism being true. We can build appliances that work on electricity and magnetism. We can build radios, cell phones, computers and space ships and do all sorts of cool things.
Second, let us compare Darwinism against other possibly wrong theories like various alternative energy theories. For example, if we can rectify the zero-point energy that is present in all space via a Josephson junction diode, we could solve the world’s energy problems. Some will say this violates the 2nd law, and others will say the 2nd law doesn’t apply to that situation, we’re merely redefining the system boundaries. Whether the Zero-Point-Energy Rectifier is possible, we at least can define the benefit: unlimited usable energy. Similar considerations would hold for condensed matter nuclear fusion or other exothermic condensed matter nuclear reactions. Whether the ideas are true or false, we can at least define the benefit if they are true. We cannot do that for Darwinism, because there is no scientific benefit if Darwinism were true.
Some will say Darwinism helps us understand anti-biotic resistance. Not really, the pre-Darwinian conception of natural selection by the creationist Blyth more accurately describes evolution of anti-biotic resistance whereby the species remains essentially a discrete entity that is distinct from others even after microevolution (i.e. E. Coli is still E. Coli after microevolution). The pre-Darwinian, Blythian conception of natural selection is arguably a better evolutionary theory by postulating created kinds evolving within limits.
Darwinism implicitly argues for genomic advance and progress, whereas real natural selection follows Behe’s first rule of adaptive evolution, namely, selection will generally favor loss of function, not formation of new function. So its really kind of hard to imagine Darwinism being true since it is so obviously false, but I’ll try for the sake of this discussion…
Some will say, the assumption of common descent helps us do medicine better. Wrong, comparative anatomy and physiology works whether one assumes common descent or common design or convergence. Comparative anatomy and physiology were promoted by creationists prior to Darwin. In fact common design may work better since we actually might try doing medical testing on ancestrally unrelated species that have similar design and physiology for certain functions.
Thus Darwinism does little if any to advance technology, medicine, or science, in fact it hurts science by discouraging participation by people who reject Darwinism.
Third, what are the philosophical benefits if Darwinism is true? Well according to evolutionary biologists Thronhill and Palmer, the phenomenon of rape is a selectively favorable trait in males. According to evolutionary biologist David Buss, the propensity to murder is also selectively favored. Thus tyrants like Ghengis Khan who murdered and raped are the most virtuous individuals in the world of Darwin…
So that is what we “gain” if Darwinism were true: no increase in technology, and the propensity to rape and murder are seen as selectively favorable traits. Darwinism can become a justification for eugenics and enslavement and denigration of others (as was done in the 19th century). That is what we “gain” if Darwinism were true.