There are always other angles to any story. He could be into very confidential matter or top national security concern. We won't know unless a report is provided to the public

If that is the case he could ask that his testimony not be done in a public forum, or that any documents pertaining to his involvement in the investigation be redacted. Asking for immunity means that he wants to be protected from prosecution which implies there is criminal wrong-doing on his behalf.

Actually, he'll be PRESUMED innocent. Generally different than actually BEING innocent, especially when you're asking for immunity.