If there were no afterlife and death means nothingness, there would be no retribution or reward, no causation of the past, present, and future, no transmigration in the Six Realms (celestials, asuras, humans, animals, hungry ghosts, and hells).
There would be no need to study Buddhism. We should by all means enjoy our lives thoroughly.
However, such is not the case with life and the universe. Karmic retribution, causation, and the cycle of rebirth all exist.