Moles leave their homes below the ground very rarely. In fact, some never do!
When they do leave the ground, it's usually to look for food. They primarily eat worms, but if they can't find any in the ground and they've grown hungry, they'll leave the ground to look for food on the surface.
They can also come out to collect nesting material when they're creating their nest. There's a similar pattern of behavior with young moles, which leave their mother's nest and leave the ground to look for a new home.
When they do leave the ground for these reasons, it's usually done at night. During the day, they're even more vulnerable than they are during the night. Their primary concern, after humans, are predatory birds like owls and falcons.