The research field of online informal English learning has revealed associations of various informal digital English activities and second language vocabulary development. However, most of these studies have regarded digital resources as uniform entities when investigating their potential for vocabulary development and have failed to consider learners’ idiosyncratic interaction with the resources driven by self-defined purposes of use. Informed by the uses and gratifications theory, this study explored how three purposes of extramural digital experience (entertainment, socialization and information) relate to vocabulary knowledge, based on the survey responses from 322 undergraduate Chinese EFL learners and their receptive vocabulary knowledge. PLS-SEM analysis uncovered differential associations of the three media use purposes with receptive vocabulary knowledge. The study also revealed that the associations between the purposes of informal digital activities and vocabulary knowledge differed depending on whether the vocabulary was high frequency or low frequency. Additionally, it was found that the strategic use of digital resources, in terms of cognitive attention to and processing of lexical information that are facilitative of vocabulary learning during and/or after the interaction, played a significant moderating role in the relationship between digital activities for information purposes and receptive knowledge of high-frequency vocabulary. The findings highlight the importance of considering media use purposes in future research and pedagogical practices.