Halliday & Matthiessen (2014: 639):
Thus clausal ellipsis/substitution occurs typically in a dialogue sequence where in a response turn everything is omitted except the information-bearing element. … A clause consisting of Mood only, such as I will, could equally occur in either the yes/no or the WH- environment; typically, in a yes/no environment, the focus would be on will, which bears the polarity (‘Will you ... ?’ – I will.), whereas in a WH- environment, the focus would be on I, which carries the information (‘Who will ... ?’ – I will.).