While researchers in recent years have exhibited a burgeoning interest in studying writing practices beyond formal contexts, the purpose of informal writing appears to be constrained primarily to recreational activities or interpersonal communication, thus overlooking its potential impact on enhancing formal writing competence. The present qualitative study aims to reveal L2 student writers' perspectives concerning the influences of informal writing on formal writing and writer development in L2 writing contexts. Informed by a dual perspective of cognitivism and social constructivism, this qualitative study collected interview data with 26 university students and their writing samples. This study reveals that informal writing (1) has the potential to improve learners' L2 writing proficiency in many aspects; (2) has both beneficial and detrimental effects on formal writing; (3) can be an emotional regulator and emotional trigger; (4) can influence writers' learning processes and learning outcomes; and (5) can affect L2 writers' personal growth. This study contributes to our understanding of the learning potential of informal writing in second language education.