Express.js is a web application framework for Node.js that provides a robust set of features and tools for building web applications and APIs. Its purpose is to simplify the development process by providing a minimalistic and flexible framework for creating server-side applications.

Key responsibilities of ExpressJS developers include:

• Designing and developing robust and scalable web applications using Express.js.
• Implementing and maintaining RESTful APIs for client-server communication. 
• Integrating with databases, such as MongoDB or MySQL, to store and retrieve data.
• Implementing authentication and authorization mechanisms to secure the application.

Some advantages of using Express.js are:

• Simplified and flexible development process due to its minimalistic nature.
• 2. Efficient routing and middleware handling for handling HTTP requests.
• 3.Simplified and intuitive API for easier code readability and maintenance.

Yes, Express.js is suitable for building both small and large-scale applications. With its flexible and scalable nature, Express.js can accommodate the needs of various project sizes.

Express.js is primarily designed for building web applications and APIs. However, it can also be utilized for other purposes such as creating web servers, proxy servers, and even developing command-line tools.