Purpose is about discovering you and then getting there.
Imagine that you're headed to meet a friend for coffee. En route, you hit a roadblock. The authorities have closed the freeway because of a spill. At that point, you might just call your friend and cancel or reschedule for another time.
Imagine, by contrast, that you encountered this road block on the way home. Likely, you'd look for an alternative route and, even though it took longer than you'd planned; you persevere until you arrived home. It is harder to dissuade us from going home.
Following this analogy, some people find home by following the green lights. This is a strategy that might result in making good time. It is less obviously a strategy that will get you home. In fact, you can try it some day. Some day when you are away from home, drive back. If the green light is an arrow, turn left. If you encounter a green light, go straight. No green light? Turn right. Just keep this up for as long as you had expected to take to get home and see where you get to.
A purpose should feel less like a coffee date, vacation, or trip to work than a trip home. You can be sure to encounter obstacles. At that point, you can give up and start sleeping in the Laundromat, never quite feeling settled or content. Or you can convince yourself that the green lights brought you to exactly the place you should be. Or you can persevere.