Answer:
In English, "hi" is an informal way of greeting someone. If you were at a job interview, you wouldn't start by saying "hi", because it's informal. "Salut" works the same way in french, as it's only in a relaxed setting with people you know.
"Hello", on the other hand, is more formal in English, and the same is with "Bonjour". "Bonjour" can be used with anyone, but the difference is that it can also be used with people you've just met, or in a formal setting, just like saying "Hello" in English.