Roofs are often the last thing on anyone's mind, that is until there is a problem. When a roof starts letting water in it's often caused by one of two things. Either recent adverse weather has caused damage to one or more parts of the roof; or your roof has reached a stage where areas are starting to deteriorate.
Typically, by the time you notice damp patches on your ceiling, your roof will likely have been leaking for some time. Insulation in your loft will prevent water from reaching the ceiling until the insulation has become saturated. This means the timber / beams in the loft may have been getting wet without you knowing. This is more of an issue if you rarely go up into your loft, or if your loft has little or no light meaning you don't notice any damp up there.