I cannot understand why mid point RVR is issued and what's the significance of it
It is (or was - I'm not sure) relevant because it is limiting:
The touch-down zone RVR is always controlling. If reported and relevant, the mid point and stop end RVR are also controlling. The minimum RVR value for the mid-point is 125 m or the RVR required for the touch-down zone if less, and 75 m for the stop-end. For aeroplanes equipped with a roll-out guidance or control system, the minimum RVR value for the mid-point is 75 m.
Note: “Relevant”, in this context, means that part of the runway used during the high speed phase of the landing down to a speed of approximately 60 knots.
source: JAR-OPS 1.405 - 1.0.5 (f)
The significance, I suspect, is because it's useful to be able to see where you are going when moving at high speed along the mid section of the runway and in manual control of steering.