Why did historical grammarians study dialects and spoken language as well as literary texts?

Historical grammarians and scholars of dialectology studied spoken language and dialects alongside literary texts for several key reasons:

• Understanding Linguistic Change: In the 19th century, as historical and comparative linguistics flourished, scholars recognized that studying the evolution of language required observing how archaic elements persisted or transformed within regional dialects. By examining these variations, researchers could better trace the development of languages, such as how Latin vowels and consonants shifted across different Romance languages.1
• Documenting Diversity: Dialect studies were driven by the need to record linguistic data that was often absent from written records. Because most nonstandard dialects were not represented in writing, they risked being lost to history. Scholars sought to capture these variations—which arise from geographic or social isolation—before urbanization, mass communication, and social mobility led to their disappearance.23
• Scientific Rigor: Early dialectologists, such as fieldworkers, gathered data through direct conversation with speakers, particularly those with minimal literacy, to capture the most authentic, localized forms of speech. This empirical approach allowed for the creation of linguistic atlases, which provided a more accurate, granular view of language distribution than the artificial homogeneity often attributed to ancient or reconstructed languages.123