Automatically resizes your image to make it fit in your responsive design.
Optionally set up breakpoints for different browser dimensions in order to display different images for different devices.
For example you can set up a high resolution image for desktop browsers and a cropped version which focusses on some detail in the smaller image for smaller devices like tablets or mobile phones.
This way your visitors don't need to load very big high resolution images while they are surfing on a low mobile data connection.