It is needed just to save working time. This process (by command grdgen) will generate TLUPlus from ITF model. The TLUPlus already contains simplified tables for quick RC estimation. Without TLUPlus, the real tool each time (for each wire) will process ITF file. With TLUPlus, the tool easy and fast obtain the RC values.
Interconnect Technology file Contains information of Dielectric material,metal layers,Resistance,capacitance values of net....,thus when the TLU+ files are not given as we need only resistance,capacitance values only we extract them from ITF file instead of using it directly as it contains large information....,