The purpose of a 10 uF cap for the MMC5883MA magnetic sensor
Capacitors charge over the course of time and thus are often used for filters. In this case, the capacitor acts as a buffer on the input to prevent the noise from influencing the pin.
If your ground signal becomes subject to noise, what happens when the set/reset pin sees that noise? It is a very decisive signal that changes the state of operation of the device. Per the datasheet:

the SET/RESET function clears the sensors of any residual magnetic polarization resulting from exposure to strong external magnets

This is something that changes the operation of your device and it may be difficult to return to your prior operating state if the set/reset pin receives an erroneous or undesired signal.