There are some key gates where the entry criteria to go through the gate are very important.
In my mind, the key gates are initiation, requirements, and UAT.
In order to pass the first gate, it is critical that there is the appropriate budget, the key stakeholders understand the overall scope, and the key business drivers for the project are understood.
In order to pass the second gate, the requirements have been analyzed by the appropriate resources and approved by the business. In agile projects, there is a gate for each user story that is passed from the business to the development team.
The most critical gate is the gate to UAT. It is critical for UAT that the testers are focused on whether the change allows business to carry out their business functions. Prior to UAT the change is fully unit tested, system, integration tested, and performance tested. In simple terms, when the change enters into UAT it works as designed and is now being tested with real business scenarios to see how it satisfies business needs.
When there are no strong gates, the focus becomes scattered, things seem to go in circles, and it is difficult moving things forward.