Most of us at some time have been prescribed antibiotics for an infection, at other times we are told it is viral or inflammation.

The same goes for all animals, vets prescribe antibiotics for infections. Infections are caused by bacteria and we usually recognise this by the presence of pus. Pus is the body’s reaction to the presence of anything it feels shouldn’t be there. It is an accumulation of cells and fluids sent to fight off the invasion of all things foreign.
We recognise in ourselves when a splinter causes the area to fester (fill with pus) and when that erupts the body expels the splinter.
The other way these cells work is to breakdown any foreign material (and kill off bacteria) and use the normal body processes to expel the by products.
It is when the body can not cope with the infection that we resort to antibiotics. Often we are asked at the surgery- “can you give me something for my dog’s sore ears, or for my cows mastitis” There is often an underlying cause for the bacteria to be present and there is a vast number of different types of bacteria. There are also a lot of different antibiotics and many of these have been developed to specifically target certain bacteria.
This is why we often take swabs or samples from infected areas to establish exactly what type of bacteria is present. We can then prescribe the most effective antibiotic.
We also need to find the reason the bacteria has invaded the area in the first place. Back to our splinter example the obvious cause of the infection was the splinter, but in the case of dogs’ ears there are a multitude of causes, e.g. allergies, swimming in dams,  grass seeds or ear mites. Just treating the symptom, that is the bacterial infection with addressing the cause means that the animal will continually become re-infected.
High cell count milk samples for cattle are a bit different and often we will get no culture results as the cells are actually doing their job and killing off the bacteria. Dead bacteria of course will not grow and be cultured for identification. It is always worth trying to culture these samples as there are certain bacteria that cause mastitis or high cells counts that are incurable and if they are present in the herd can spread the infection to other animals.
Infections need to be differentiated from inflammation and viral infections, neither of which will respond to antibiotics.
So the bottom line is that veterinarians will prescribe antibiotics when they have established the cause and type of infection. And antibiotics are a prescription medication which means that by law the animal must be seen.
Hope this makes things a little clearer but if it has left you confused feel free to discuss matters with your vet.