According to wikipedia

‘…they feed on bacteria and can become problematic…In small numbers, the larvae are sometimes considered beneficial, as their strong jaws can cut through the hair and sludge waste in drains which might otherwise form clogs…’

As to habitat prior to humans building drains and shower stalls etc, probably stagnant water, but I’m just speculating

edit according to wikipedia ‘…live in aquatic to semi-terrestrial or sludge-based habitats…’