ArcGIS is a comprehensive system that allows people to collect, organize, manage, analyze, communicate, and distribute geographic information. As the world's leading platform for building and using geographic information systems (GIS), ArcGIS is used by people all over the world to put geographic knowledge to work in government, business, science, education, and media. ArcGIS enables geographic information to be published so it can be accessed and used by anyone. The system is available everywhere using web browsers, mobile devices such as smartphones, and desktop computers.

In general, people use ArcGIS because it helps them to:
Solve problems
Make better decisions
Plan successfully
Make better use of resources
Anticipate and manage change
Manage and run operations more efficiently
Promote collaboration between teams, disciplines, and institutions
Increase understanding and knowledge
Communicate more effectively
Educate and motivate others

there is a dire need for more properly trained GIS officials, and easier access to products.
To avoid the what do I buy question, I instead ask the question why do you use Esri ArcGIS. From my experience its slow to navigate around maps and quite verbose to program in causing quite a lead time in development. Esri is also the most expensive product on the market, and i believe also the most popular. But why do people continue to use it, when other systems like CadCorp have the same functionality if not more, seem to be more responsive, can import Esri data and cost less.