To believe in rationality (the empirical process of science) is to establish the virtue of logic. This tends to defend against mythical narratives, such as God/Heaven and cult promises of the super-sensory variety.
However, to dedicate yourself to a purpose (a grand common task) from which we can achieve more often requires irrational motivations: to go to the moon is silly. Still, the scientific byproduct has been eminently practical.
Leaving room for inane thick desires is often irrationality, but that does not mean it does not serve a rational purpose retroactively.