Having related projects under a common program makes it easier to understand how the project fits together and what impact they have on the company goals. A program works towards the same goal. So if at some point a project needs more resources or people it makes it easier for project managers and project team members to understand why people need to transfer to other teams. The purchasing power of a program is greater than that of any single project within it. A program can get discounts on tools, infrastructure, or services when more projects are using them.