People can "believe things they also see as lacking purpose," if you mean that they believe the thing lacks purpose, e.g. I might believe that the stone lying on the ground over there is pointless. But the fact that I hold the belief is not pointless. In the case of the stone, I hold the belief about it in order to navigate the surrounding terrain.

And in general, as onlytheseekerfinds noted, beliefs are costly by impeding alternative beliefs, and they also have a cost even in physical energy, since it consumes resources to express them, even mentally. So no one would hold a belief unless there were some corresponding benefits to outweigh the costs. Robin Hanson talks about this a bit here.