Service dogs can present unusual challenges for treatment of routine issues because they are working dogs. For example, the standard treatment for diarrhea in companion dogs is a 12-24 hour fast or a smaller, bland diet for a few days. However, if you're on an airplane or in a hotel, this may not be a viable option. OTC treatments can make a huge difference in such situations.
Note: This is information I've gathered for use with my own dogs, in consultation with my veterinarian. I am not a veterinarian (nor even a veterinarian technician), so consult your own vet before taking my examples as gospel. Also, remember that using OTC treatments too often can mask more serious problems; don't use them unless you have to!