So that all aspects of the project are looked at and considered.
If you are deciding what new database server to buy for an application you’ll want to talk to the systems admin so that you know that the hardware fits within the companies support standards, etc. You’ll want to talk to the database administrator to ensure that the new server has the correct version of the database, supports the correct database platform, that it has enough memory and CPU power to handle the database load. You’ll want to talk to the developer to find out what the load requirements are. You’ll want to talk tot he business unit to find out what the SLAs for the system are.
If you guess on any of these points, and you guess incorrectly the project will probably fail.