What is the reasoning behind the Interface Segregation Principle?

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

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One reason is that having many interfaces with a minimal amount of methods for each one makes it easier to implement each interface and to implement them correctly. A large interface can be unruly. Also, using a focused interface in a scenario makes the code more maintanable because you can see which facet of the object is being used (e.g., an IComparable interface lets you know that the object is only being used for comparisons in the given scenario).