There are any number of valid reasons you could reasonably give for adhering to a sound project management methodology. Personally, I distill all of these into a single reason: customer satisfaction. That is the only reason of importance.
In any project there are three customer roles you must satisfy. They may be all rolled in to the same person, or there may be spread into any number of people or agencies, but they can all be categorized into the three roles nonetheless. These are the sponsor, the regulator, and the end user. Managing a successful project is all about satisfying the needs of these three roles.
The sponsor is that person or agency that has commissioned the project and to whom the project manager is directly responsible. The concerns of the sponsor are those that are typically cited when you ask someone what are the responsibilities of a project manager:
Concerns of the Sponsor:
There's a lot involved in meeting these concerns, and that's what most of the project management literature you read will deal with. But we're talking about goals here,
The regulator is any outside agency that imposes conditions upon your project. For example, you may be doing a project for a financial or insurance company which may need to meet certain legal requirements. A point-of-sale system needs to collect tax properly. Or you may simply need to conform with the development standards of your IT department. One function of a Quality Assurance department (if you have one) is to ensure regulatory compliance.
Concerns of the Regulator:
These are concerns that should concern you as much as the sponsor or the end user. So if there are no specific regulatory agencies or QA authorities you must satisfy, then you must take on this role yourself.
The end user are the persons or agencies most directly affected by the quality of the project.
Concerns of the End User:
Note that unless you address all of these concerns, your project will fall short of total success. It may not be a failure, but it won't be well received.
Introduction Vision, Mission, & Values