Free ranging chickens are never totally safe from predators. If you free range, you take that chance, even if you are standing there with them. There is no guarantee that you will lose one but there is no way to guarantee that you will not.
Sometimes a good rooster will help you out. The rooster is not going to stop a lot of predators, but he usually is on the lookout, for hawks especially. Part of his job is to warn the hens. A good rooster will put himself between the hens and the predator, sacrificing himself to give them a chance to get to safety.
Not all roosters are good. My current one leads his hens to safety instead of putting himself between the danger and them. His replacement is about 15 weeks old, so his days are numbered.
Often, with an all hen flock, the dominant hen will take over many of the rooster's roles as far as flock management. That includes looking out for predators or even putting herself in harm's way if necessary. But again that does not always happen.
Do you need a rooster for them to free range? No, not really. You only need a rooster to fertilize eggs. Will your hens be safer if you have a rooster? Probably a little bit, but it really depends on the rooster.