From a religious point of view, freedom is important in ethics for two main reasons.
First, when we speak of freedom, we are saying that people are capable of making decision without coercion. They are able to choose to do good and are able to choose to do evil. This is fundamental to any discussion in ethics. For example, if a person is forced to do something "immoral" or "wrong," then the ethics is compromised. To put a gun to someones head or a loved one's head makes decision harder to make.
Second, freedom also assumes that there is intentionality. In other words, people intentionally make an active decision to commit or not to commit to something when there is freedom. In the end, a person is making a decision based on their own freewill. This makes a person responsible for his or her actions.