Hi,
This minimum spacing is to gaurantee that the drain and source areas will not be shorted to each other during manufacturing of this device.
In the manufacture steps of this device, the poly is deposited over the subestrate first, next is implanting the substrate with the required diffusion n+ or p+, so if the poly was not fully covering the transistor active area, as shown in figure, a small doped channal will exist between the source and drain areas shorting them to each other.
Hope this helps,
Regards,
Shohdy