It depends. Private investigators cannot bug buildings at random. The person requesting the bugging must have legal ownership over the residence to be bugged. For example, you can bug your own house. This would be useful for an infidelity investigation. You could also bug your business. This would be useful for catching dishonest employees in the act. However, you cannot legally bug the house of a romantic interest or ex. Additionally, there are certain rooms where people expect a reasonable degree of private. These cannot be bugged at all. This would include bedrooms and bathrooms.