Not appearing to be giving the finger to congress, as it looked when he failed to show up to the last set of hearings? Pissing off congress is rarely a good idea.
I'm not sure why Congress thinks it can at the drop of a hat order a CEO to participate in their media theatrics, and then pitch a fit when the company says "he/she is busy, we're going to send our top legal expert, who should know about the stuff you're concerned with (law) anyway."
That means what you think you know from things you read in the media might actually not be as truthy as what is said when someone has to sit down before a body that has the power to criminalize dishonesty.
And its an important tool for government. There are very public scandals that the people want answers for. You may say it fits nicely with the expectations of office that elected officials might get to the bottom of things that may effect large groups of their constituents.