Roof repair is difficult to teach. There are many great roofers, but only a few who can track leaks effectively. To be a good leak tracker you must have a good understanding of building envelopes and also be a honed detective.
It is important to ask lots of questions to try and establish the history of the roof. Is this an ongoing issue? Does it only leak in the winter? How old is the roof? These are a few examples.
Many of the leaks we are called out to are not leaks at all but rather problems with the building envelope. Lack of venting can cause condensation. Humidifiers set to high in the winter can cause large amounts of moisture to be trapped and freeze in the attic. A common problem we see is overstuffed attic insulation causing trapped moisture in the attic to migrate to the ceiling below.
Leaks can also come in at windows, siding and the foundation. A good leak tracker must look at all aspects of the building not just the roof. Most real leaks are caused by poor installation. The proper way to repair a roof is to remove the shingles and re-shingle the affected area. Many companies will tar the leaking area. This may stop the leak in the short term but is rarely an effective repair. The tar also makes it very difficult for the next leak tracker to solve the problem.
If a leak tracker offers to solve the leak by tarring over the problem you should consider getting another opinion based on the understanding of how to do a proper leak repair.