A few situations that might suit the use of X10 devices:
If you have a detached garage, shed or other outbuildings, it can be very convenient to be able to switch the lights on and off from inside the main building as well as from the outbuilding. In a normal wiring scenario, two-way switching and a three-core cable run from the house to the outbuilding would be required. This would be in addition to any existing cabling installed already.
Suppose you have a detached garage 20 metres away from your house. You already run a heavy armoured cable from the house to the garage and do not wish install more. With an X10 lamp module in the garage you can control the garage lighting from anywhere; no extra cabling is required. In addition, you can install wireless X10 switches without running cables, perfect for multiple doors.
Coupling room lights
If you have merged two rooms in a house to make a single large room, you end up with one long room with two doors and seperate light switches. What you really want is for both switches to operate both lights.
No problem; just replace the switches with X10 modules set to the same X10 unit code and either switch will control both lights.
Most houses have a basement or attic is controlled by a small switch in the room. To get any light in the room, you would typically need to walk in and wander round in total darkness looking for the switch.
However it would be much easier to install lamp socket module and switch on the light using an X10 remote or controller before entering the room.