What is the purpose of comparing spots from unknown substances with known substances on a chromatogram?

A high-scoring answer should explain compare spots from unknown substances with known substances on the same chromatogram. Use method first, observation second, conclusion last: name the relevant test or measurement, state the observation, and then connect the result to Chromatography.

This is correct because spot and solvent-front distances is the evidence expected for chromatography, and Rf or separation evidence is the result that supports the conclusion. A complete answer should use the approved objective wording, include the relevant evidence, and avoid unsupported identification claims.