A flat in-room measurement almost never sounds natural — our ears expect some high-frequency roll-off (due to air absorption over distance in natural listening) and a slight bass boost (to compensate for the ear's reduced bass sensitivity at moderate volumes). Target curves account for these expectations.
Common target curves: the Harman curve (scientifically derived from listener preference studies — slight bass boost, gentle treble roll-off), the B&K curve (flat to 2 kHz then gradual roll-off), and flat (accurate but often perceived as bright). Most room correction systems let you customize the target curve.