A safety pilot is required when practicing instrument flight under simulated conditions (under the hood), to provide a lookout for other traffic. Operating under an IFR clearance would not provide separation from VFR traffic.
91.109 Flight instruction; simulated instrument flight and certain flight tests.
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(b) No person may operate a civil aircraft in simulated instrument flight unless— (1) The other control seat is occupied by a safety pilot who possesses at least a private pilot certificate with category and class ratings appropriate to the aircraft being flown.
(2) The safety pilot has adequate vision forward and to each side of the aircraft, or a competent observer in the aircraft adequately supplements the vision of the safety pilot; and
(3) Except in the case of lighter-than-air aircraft, that aircraft is equipped with fully functioning dual controls. However, simulated instrument flight may be conducted in a single-engine airplane, equipped with a single, functioning, throwover control wheel, in place of fixed, dual controls of the elevator and ailerons, when—
(i) The safety pilot has determined that the flight can be conducted safely; and
(ii) The person manipulating the controls has at least a private pilot certificate with appropriate category and class ratings.
(c) No person may operate a civil aircraft that is being used for a flight test for an airline transport pilot certificate or a class or type rating on that certificate, or for a part 121 proficiency flight test, unless the pilot seated at the controls, other than the pilot being checked, is fully qualified to act as pilot in command of the aircraft.