The details of the grant review process vary between the various grants available, but you will usually be working with a number of collaborators throughout. It is important to be well-versed in the criteria for and goals of the grant, and if there is a scoring rubric available, follow it carefully.
It is important that there be a good outlook for the utility of the research or other project seeking your grant. However, you should also keep in mind that research with a well-anticipated outcome is unlikely to much advance a field of knowledge---ostensibly, the goal of research.
A grant review does not only serve to determine which projects receive funding and which do not; reviews also provide important feedback for the projects not chosen, and direction to the projects that do receive the grant. Thus, feedback on elements of a project, especially which segments are good and which require more attention, is key to a well-written grant review.