Is it necessary to have the Catalytic Convertor?

Well, it's a violation of the Federal Clean Air Act since 1988 to remove or tamper with them.
Used to be that the fines would only be assed if it was a shop that did the work, which changed about in 1998.
"Violating the EPA rules is a breach of federal law since noncompliance is likely to increase the amount of pollution coming out of the vehicle. Penalties for violations by individuals, service or repair shops or fleet operators can be up to $2,500 per violation. (Each improper installation is considered a violation.)

Additionally, new-car dealers can be penalized up to $25,000 per violation. Any person who causes a violation could be subject to the same penalty as the technician. Failure to maintain complete records or submit reports may also result in a violation."

Plus, if your state is one that does inspections it's an automatic failure if it's not there. For instance, NYS added this - New York State Environmental Conservation Law regarding catalytic converter installation mandates a minimum penalty of $500 for a first violation, and up to $26,000 for each subsequent violation. So, say the vehicle has 2 front units and 2 rear units if you remove them, you get fined $500 for one and then $78,000 for the other 3.