We tend to trust even expect future events that are similar to past events. The more proof we have that we have that things will not change, the more we expect in a familiar future. We are creatures of statistics. The more samples, the more we believe the trend will continue. I expect there to be another spring.
What about hope of something that we have not personally witnessed. We have records and other individual’s witness that certain actions yield certain results. Here we come into trust of the individual or the record. We still almost fully rely on our personal experiences. We find it difficult to trust another’s witness when it varies in the least degree from our interpretation. Trust can overcome. Is there anyone you trust more than yourself?
The final form of hope comes into play when we attempt to trigger a result from an action that we have never taken, and has limited or no comparisons in-which can judge an outcome. True pioneers of the unknown take bravery and skill to be successful. In most situations one of the previous forms apply, but there are those wonderful moments when we can truly step out into hope and reap the rewards.

Why do we need hope and when is hope needed?

Every choice has a result. Every action has an opposite reaction. We must have hope that the reaction we are seeking will result from our action. If we do not have hope we do not, do. We are passive when in doubt. We do not tempt reactions. This means that all actions stem from hope. We are processors of what we have seen, what we are experiencing, and what we expect.
In matters of force the reaction is equal. In matters of choice, a divergence of path may be subtle, but its results long lasting. A baseball thrown in deep space will continue on its path until acted upon by a new force. We naturally do not tempt reactions, and thus maintain the same path.
We are sentient, sensitive, agents unto ourselves, only acting for a reaction. If we hope to have reaction, we must have action. Weather automatic and trusted or new and risky, we must always act.