Decision that a certain project is worth financing has to be made very early. Otherwise, it might not start at all. This means that the decision maker has to be supplied with enough information to provide a level of confidence that the risk is reasonable and worth taking. This information includes high-level requirements, a high-level design, and even prototyping or some other proof that the available technologies can be used for the successful implementation. Based on this data and market research, the project advocates estimate the level of effort, expenses, potential income, and future profit—the mother of all goals.
Even before the project gets a green light, the most critical for the success of the product features are identified, presented in a form that can be communicated to the future customers, and discussed or even tested with them. If the team includes people who did something similar in the past, it definitely helps to shortcut the decision making.
The purpose of this phase is to present the future system in a form that could be understood by all participants and potential customers.