The purposes of Analysis & Design are:

1. To transform the requirements into a design of the system to-be.
2. To evolve a robust architecture for the system.
3. To adapt the design to match the implementation environment, designing it for performance.

An analysis mechanism represents a pattern that constitutes a common solution to a common problem. They may show patterns of structure, patterns of behavior, or both. They are used during analysis to reduce the complexity of analysis, and to improve its consistency by providing designers with a shorthand representation for complex behavior.
Mechanisms allow the analysis effort to focus on translating the functional requirements into software concepts without bogging-down in the specification of relatively complex behavior needed to support the functionality but not central to it.