Why is it important to minimize impurities when doping a semiconductor?
Semiconductors are based on impurities.  They just call it doping.  The addition of group 3 (Boron group) or group 5 (Nitrogen group) impurities create a hole or an extra electrno, respectively.  These form charge carriers which are necessary for a semiconductor to be used in a circuit.  However, the dopants need to be added in highly controlled and precise amounts to get the necessary mobility.  The silicon also needs to be highly pure for regular electronic response, for regular electronic properties over arbitrary distances.  Another important consideration is that the dopant allows some degree of tuning of the electrical conductivity of the semiconductor.

Also, highly pure crystalline silicon  fractures nicely along the crystal planes, meaning it can be cut with high precision.  Since most mass-produced electronic processors are made on 12- or 16-inch silicon wafers with dozens of devices manufactured at once, and initially all physically connected, the ability to precisely excise each tiny circuit is critical for high through put and yield in manufacturing.