Why do we need to know power factor in electrical loads? How does it affect the loads?
Power factor does not affect loads. Power factor is an inherent characteristic of loads. If the angle of the voltage and current is zero, all of the current taken by the load is used to deliver power to the load. If the angle is not zero, that means that some energy is circulating back and forth between the source and load. The additional current causes additional losses in the generation, transmission and distribution system and wastes system capacity. That is unprofitable for the utility that supplies the power, so they typically add a surcharge to large consumers like manufacturers that operate with low power factor.

Induction motors require circulating energy to maintain the rotating magnetic field that is produced by alternating current. That can be supplied by capacitors installed near the motors. That is typically done by large industrial users. The utilities add capacitors near small customers for the same purpose.

Electronic devices cause low power factor due to distortion of the current. That can be mitigated by circuit design. Utilities and large customers can also use local mitigation equipment (harmonic filters).

This is a large and complex problem. It can not be explained completely in the question and answer format of this site. Other questions here may have helpful answers. Additional background information is available in AC power tutorials, lecture notes etc. that can be found online.