What is the Default field used for ?
The default value is the one that is automatically chosen: the "default" value. While capturing leads online, for example, the status will be set to the default value. Many fields may have default values, such as text fields, date fields, and picklists.
What is the "Responded" field used for ?
It's used to indicate that a status value indicates a positive response to the campaign. The default configuration has the values of "Sent" and "Responded." This allows you to run reports to find out interesting information. For example, if 100 emails were sent out, and 28 responses were received, we would know that the conversion rate is 28%. This allows us to evaluate the effectiveness of campaigns. Since you may have many different values, even per campaign, it is important to know what constitutes a response, versus those that are not responses, such as "Not Interested" or "Left Voicemail".
Campaign Member Status is one of the few fields that has an "extra" significance to the selected value, including Lead Status, Opportunity Stage, Case Status, and Contract Status. Each field produces some extra effect that aids in improving performance or signalling a change in the system. This particular field produces additional information on the success or penetration of a marketing campaign.