What is a wireframe sketch primarily used for?

A wireframe sketch serves as a foundational blueprint for a webpage, outlining its structure and layout. This tool is essential in the early stages of web design, as it visually represents the arrangement of elements such as navigation bars, content areas, images, and buttons without delving into specific design details like colors or fonts. By focusing on the layout, wireframes help designers and stakeholders quickly conceptualize how users will interact with the site and where components will be placed, ensuring a logical flow and usability prior to moving on to more detailed design aspects.

The other options involve different stages of the design process. Creating animations pertains to interaction design, which comes later. Finalizing a color scheme is a part of visual design, typically handled after the wireframe stage, and writing content is related to the site's actual messaging, which is usually developed once the layout is defined. Therefore, the primary use of a wireframe sketch is indeed to outline the structural aspects of a webpage.