if you don’t need penicillin does it actually help you at all?

No, it has virtually no chance to help you, and most probably can only hurt you.

First, it kills indiscriminately. If you're not sick, what are you killing? Your own healthy gut flora. That's what.

Second, what if you are slightly ill? Guess what? It still probably won't help. Doctors don't just throw penicillin at you in random amounts. They prescribe a specific dose that has been shown to be effective. Having one untested dose of unknown quantity isn't going to help.

Third, when you're given antibiotics, you are told to take it over a number of days, and to take the entire amount, even if you feel better. They do this for several reasons, but one of the reasons is that, if you only kill some of the bacteria, but not enough of them, the remaining bacteria have a small chance to evolve to become resistant to antibiotics. By taking antibiotics without the guidance of a doctor, you have a small chance of making yourself even more ill with antibiotic-resistant bacteria. I want to emphasize that this is a very small chance, but unlikely things will happen when given enough chances.