What are media queries in CSS used for?

Media queries in CSS are used to apply different styles based on various conditions such as the size of the viewport, the resolution of the device, orientation, and more. This feature allows developers to create responsive designs that enhance user experience on a variety of devices including desktops, tablets, and mobile phones.

For instance, if a website looks great on a large screen but isn't as user-friendly on a mobile device, media queries can change specific style rules such as font sizes, layout arrangements, and image sizes for smaller screens. This adaptability is crucial in modern web development, where users access content from an array of devices with differing capabilities.

The other options focus on functionalities that are unrelated to the specific purpose of media queries. Increasing page load speed involves optimizing resources and is not directly tied to CSS styling. Animations are created using CSS transitions and keyframes, not specifically through media queries. Similarly, defining color schemes relates to setting colors and not adjusting styles based on the viewport or device conditions. Thus, utilizing media queries is essential for ensuring that a website is both functional and visually appealing across all devices.