It is said that a Tile roof lasts forever.
This is not entirely true. The Tiles are made from Clay or Cement so they do last almost indefinitely, but a "Roof" or (Roof System) consists of more than just that top layer of material. The Tiles, or "top layer" of a roof or (Roof System) serves more than one propose.
1) Of course the top layer is the aesthetic layer, but also it protects the sub layers i.e. Tar Paper from the elements like sun and rain.
2) If done correctly, the "top layer" along with the flashings (i.e., Valleys, Pipe and Wall Metal) can also keep all of the water away from the Sub layers or Tar Paper.
That said, if a Tile becomes cracked or slips out of place, or the Flashings such as Valley, Pipe and Wall Metal were improperly designed, the Tar Paper can hold the water back for a number of years but eventually will rot and allow the water to penetrate into the house.
Obviously it is very important to design these areas well where flashings are needed to integrate the roof tiles with different angles, walls, pipes, chimneys, etc...
The job of designing a roof system that will last is a job that takes skill, experience, imaginative thinking and a degree of intelligence, of which some workers just don't have.
Don't take for granted the complexities of quality roof repairs, It is very important in this industry for a well meaning home improvement contractor to supervise every job.
San Juan Capistrano Roof Repair