It links a cause you are curious about to the result you can see, and in these lines it nearly always sits in a question with 뭘, 얼마나 or 무슨: what did he eat, that this happened; how high is it; what on earth did they talk about. The visible half is the outcome and the missing half is the cause you are fishing for. Compare 아서 and 어서, which simply state a cause you already know.
Register: casual and curious; very common in captions