Why would you want all projects to be proposed in a uniform way?  What would you suggest as information that must be available for all projects?

Project proposals need relevant facts/data, stated purpose, means to track or find out if said purpose is being achieved, financial impact and benefits/outcome. If not proposed in a uniform way, proposals would be heavy with justifications and reasoning why the project should be approved without any regard to available resources, cost of said resources, impact to current production environment, staff and operations.

What is the importance of having a target mix before starting to approve projects?

This allows buy-in from as many staff as possible and gives important input/feedback.