Work difficulty and team member growth
In addition to allocating the correct team composition to projects. A common balance you need to strike as a leader is in assigning projects (or advocating for project leads). As someone trying to get work done, your tendency is to give it to your strongest, most senior members. This makes sense on paper, but it can lead to people feeling bored with their work / jealous of those given more challenging work. Further, your staff will not being growing, at least not to their potential, with work that is always right in their wheel house / level. Striking the balance between priorities, team morale, and culture is crucial.