This paper, based on the exploration of crypto-functions of formal markers in English (Dong, 2016), preliminarily attempts to explicate the motive of their crypto-functions in systemic functional framework through a diachronic approach. The study firstly claims that formal markers can be treated as one of multiple linguistic expressions deployed for constructing the experiential phenomenon. Then the study assumes that the motive of crypto-functions of formal markers can be revealed from the diachronic conventionalization of multiple linguistic expressions towards formal markers in their process of experience construction. And such an assumption has then been tested with the distribution frequencies of formal markers in Corpus of Historical American English (COHA). It is found that, with the diachronic increase of the distribution frequencies of formal markers in COHA, the diachronic conventionalization of multiple linguistic expressions towards formal markers does exist, and that such a conventionalization is then attributed to language chunk composed of formal markers together with other following elements and renders formal markers prefabricate potential, thus resulting in their crypto-functions.