BMF055 is nothing more than a BMG160, BMA280, BMM150 and SAMD20J18RevB microcontroller in a single package.
The choice of using a crystal or not in the microcontroller is up to you, and the benefits of a more accurate clock vary per application.
In the case of sensor fusion, since the gyroscope data is integrated, a more accurate clock leads to a smaller error. the temperature characteristic of the crystals oscillators are also usally far better than the internal RC oscillator over the temperature range, leading to a more stable accuracy over temperature changes.
