<p>The Interface Segregation Principle (ISP) says that many client specific interfaces are better than one general purpose interface. Why is this important?</p>

ISP states that:
Clients should not be forced to depend
  on methods that they do not use.
ISP relates to important characteristics -
cohesion
and
coupling
.
Ideally your components must be highly tailored. It improves code robustness and maintainability.
Enforcing ISP gives you following bonuses:
High
cohesion
- better understandability, robustness
Low
coupling
- better maintainability, high resistance to changes
If you want to learn more about software design principles, get a copy
of
Agile Software Development, Principles, Patterns, and Practices
book.