Most environments are full of some type of electromagnetic interference (EMI). EMI can wreak havoc, or at a minimum, cause a circuit design to experience less-than-optimum performance. Designers may be able to create a very simple EMI filter to keep EMI from affecting their circuit performance level.
However, in many cases, a more extensive EMI filter design might be necessary to remove, or at least mitigate EMI so as to not impede achieving the desired performance goals.