The mind, though variegated by innumerable desires, acts for another (the Purusha), because it acts in combination.
In plain words: The mind is full of countless desires, but it is made of many parts working together. Things made of parts exist for something beyond them, so the mind works for the Purusha, the true Self.
The mind is a compound of various things and therefore it cannot work for itself. Everything that is a combination in this world has some object for that combination, some third thing for which this combination is going on. So this combination of the mind is for the Purusha.
Namaste.
A calm guide to life, the mind and the old teachings.