The common bat is a generally inoffensive creature. Small and unobtrusive, the most they are is annoyances in your attic or sites on the road.
Common bats do have a tendency to swarm, but even swarming, they are not generally harmful to humans. Swarming bats can extinguish torches and disrupt spellcasting, but aside from this obstruction, they do little. Their environment can also be dangerous: bat guano that accumulates in caves attracts rot grubs and carrion crawlers, and is somewhat of an overpowering stench.
Bats have resonance in many legends and myths as creatures of the night and of the secret and eerie world therein. Vampires are especially known for associating with bats, but many creatures, especially in the shallow underdark or the deep forests, live near bats.
Bat wings, fangs, and ears play a role in many alchemy recepies. Guano, the droppings of bats, is also valuable for fertilizer and as an explosive agent for fire magic.
Wizards may be found in the company of bat familiars, and these creatures are said to make their masters keenly aware of the world around them.