One way to help students engage in higher-order thinking is through scaffolding, which can be defined as support that allows students to participate meaningfully in and gain skill at a task that is beyond their unassisted abilities. Most research on computer-based scaffolds assesses the average impact of the tools on learning outcomes. This is problematic in that it assumes that computer-based scaffolds impact different students in the same way. In this conceptual paper, we use activity theory and the theory of affordances to build an initial theoretical framework on how and why K-12 students use computer-based scaffolds. Specifically, we argue that affordances and motives drive how and why K-12 students use computer-based scaffolds. Then we examine empirical studies to gather preliminary support for the framework. Implications for research on and the design of computer-based scaffolds are explored.