Answer: Graphite is a soft lubricant and good conductor of electricity as it pi bonds.
Explanation: Brushes collect the current due to the induced emf from the armature coils. When a brush is at any particular commutator segment, it shorts out that particular coil and draws current from the rest of the coils.
If the brush is not exactly at the particular segment, say one half is in one segment and other half is in a different one, then it draws current in that proportion.
But the thing remains that, at any particular moment of time, the brushes are always shorting one coil. Now as it is a short circuit, if emf is induced in that coil, huge current will flow. To prevent this, brushes are positioned in such a way that the coil being short circuited has no induced emf in it.