It doesn’t mean “fuck all”. If you’re vaccinated you are less likely to have a large viral load replicating in your respiratory cells, so you’re less likely to be breathing out as much of the virus if you have it and don’t realise you have it.
Don’t forget, a negative test only means it’s unlikely to have Covid, but it could be a false negative. So you could still have Covid if you test negative therefore being vaccinated in that situation is better. It doesn’t meant fuck all, it’s literally spreading less of the virus.