Generally, criminal defendants are not supposed to even be privy to grand jury proceedings in which they are the subject, if they haven't been arrested. Anyway, it would make no sense to call Trump to testify if they've already decided not to indict him. So it's pretty safe to assume the prosecutor is giving him a chance to give testimony that could be exculpatory, at least in theory.
Generally, criminal defendants are not supposed to even be privy to grand jury proceedings in which they are the subject, if they haven't been arrested. Anyway, it would make no sense to call Trump to testify if they've already decided not to indict him. So it's pretty safe to assume the prosecutor is giving him a chance to give testimony that could be exculpatory, at least in theory.