I have been an ER nurse for a few years, so I'll give you my take, if I'm understanding your question correctly.
Basically, treating an infection in the ER without cultures, is usually due to time sensitive needs.
If the person come is very sick looking and with high WBC count, there are a few types of infections such as bacterial meningitis, that will kill you fairly quickly if not treated. As you probably know, cultures of any kind will take at least 24 hours. The patient may not have that much time if he is left untreated. This is especially important for very young infants with fever, antibiotics must be started ASAP, we cannot wait for cultures to rule out viral vs. bacterial infection.
Low glucose in cerebrospinal fluid can indicate bacterial infection, so broad spectrum antibiotics will be started. Blood cultures are read at 24 hours, but are usually negative, then again for final read at 48 hours.
Certain infections will cause specific symptoms and will be treated as such until proven otherwise