For a condition such as an ear infection -- is the ear drum intact, or did the repeat infection involve the ear drum and rupture it this time? Is this ear infection caused by yeast or bacteria? What about a fox tail (grass awn) or other foreign object? Last time it was a bacterial infection, what if there is an infected polyp this time? Your veterinarian will want to assess the ear anew and prescribe appropriate treatment. To simply prescribe what was given last time may or may not be effective, depending on the cause of the problem.
For chronic health conditions that require medication, such as hypo or (some) hyper thyroid conditions, most vets require yearly bloodwork to make sure that the therapy is still effective. The bloodwork may reveal that the medication dosage is too high or low before clinical signs are apparent to the owner.
In conclusion, if you ever have questions about what your veterinarian is doing or why, ask him or her! Or at least ask the veterinary staff -- they are there to help assist you as well as the veterinarian.