Another reason to get a culture is that antibiotics do have risks. They can weaken other beneficial bacteria (and science is only barely beginning to understand how many of those are important to humans, let alone reptiles). But also, every use of antibiotics can contribute to breeding resistant populations of bacteria. That's a big deal on an individual level as well as on a global level. That's why you should always finish the entire course of antibiotics even after you're feeling better - to maximize the chance that you kill off all the offenders, not just most of them.
We generally consider the risks worth it if they help an animal (or a person) fight off an infection or maybe prevent a secondary one. But it doesn't do anyone any favors to just administer antibiotics without knowing whether they have any chance of attacking the problem.