When you think about your roof, chances are that the component you focus on is the covering itself. This would be the Asphalt Shingles, Wood Shakes, Concrete Tiles etc. (Depending on what type of roof you have)
Where the roof typically has troubles however is at the transitions and penetrations. (Skylights, Vents, Chimneys, etc.) These are areas where the roofing material has to be cut. To keep water out, flashing and sealants are used.
Over time weathering, tree debris, or moss can cause these components to fail. And of course if they were not installed properly in the first place, the risk is greater.
With proper inspections and treatment, these issues can be corrected for a fraction of the cost of a major leak or roof replacement.