To make the handling Time/Aperture easier, the exposure value (EV) has been introduced.
Each step on the aperture scale corresponds to one step on the time scale. To hold the light quantity constant, you also have to change the aperture after having changed the exposure time. The graduation of both scales each corresponds to a double or a half of the light quantity.
One EV always corresponds to a certain light quantity and thus to a firm time/aperture ratio. EV 0 corresponds to aperture 1 at 1 s. If the camera permits the setting of an exposure value/EV, it will be very easy to give priority to the shutter speed or to the lens opening without having to calculate again.