Why would you use design patterns?
The answer is so simple it is almost a tautology:
Design patterns are used because they make your job easier.
Design patterns let you write better code more quickly.
However, patterns are not a panacea. Of the five phases
software development, design patterns do almost nothing in the analysis, testing, or documentation phases.
Design patterns, as the name implies, have their biggest impact in the design phase of a project.