During the flyby, DSN operators reported “a slight effect in the Doppler residuals,” meaning that, as expected, Phobos’ gravity had accelerated Mars Express’ orbital velocity very slightly. Through careful analysis of the Doppler shifting of the radio signal, Phobos’ gravity can be measured, allowing scientists to discern its mass and density — the most precise measurement to date.

via Mars Express Orbiter Buzzes Martian Moon Phobos : Discovery News.

However, the operation allowed the spacecraft to beam back an extra 200 Gigabits of observational data, including imagery of Phobos during an earlier 500 kilometer pass of the moon.