What's practical changes in each person you meet, so there isn't really a definite answer. But there are plenty of things that would normally be considered practical that INFPs can do/ apply themselves to. Common careers known for us are therapists, novelists, teachers (especially kindergarten); the insightful jobs. Just look at the INFP strengths and see where they could help society. And remember that just because a career/job isn't focused on the whole country or world (like a politician or government workers), doesn't mean it isn't practical. If it helps something or somebody, it is practical, thus it has purpose.
One more note: Yes, that even means artists have purpose; I mean of course they do! Their drawings and images fascinate the everyday body and on occasion, teach life lessons. In a metaphorical sense, they are the ENFP to a distraught INTJ.