The definition of close placement (of wisdom) by mindfulness on mind is:
An exalted knower on the path, that is contained in either mindfulness or wisdom, and which meditates by investigating the general and specific characteristics of mind.
In short: Wisdom, a discriminating awareness is placed by mindfulness on mind, to investigate its general and specific characteristics.
Focal object: Mind is synonymous with primary consciousness, and has a sixfold division according to the six sense powers, five physical and one mental.
Purpose: To eliminate any type of mind as the self, and to then also realize the emptiness of the mind itself.
The placement of wisdom on the mind eliminates mind as the basis for a self through the analysis of its impermanence, lack of intrinsic existence etc. Through this the grasping at the mind is eliminated, and one can proceed to attain cessation without having any fear.
To just meditate on the clear nature of the mind does not harm ignorance
From the Debate between Wisdom and Ignorance:
Ignorance says to Wisdom:
The meditation without wandering from clear and knowing
Is seen as the pinnacle of placement meditation.
If one does not even attain liberation
Then omniscient consciousness is a joke. 
If meditation on clear and knowing
While being oblivious to anything else,
Is meditation on mind itself,
Since the mind and myself are inseparable 
It becomes a meditation on myself alone.
As the result of meditating on me
Not only is there no attainment of liberation,
The only thing that will happen 
Is that I will become stronger, nothing else.
As we said earlier, some meditate on nothing else but the clear nature of the mind, without thinking or conceptualization. But because mind is an illusory conventional object, it comes with true-appearance, and therefore meditating on it will only strengthen true-grasping and does not benefit the attainment of liberation.