[nonameiguess]

You seem to be wondering why the act of conspiracy to commit a crime is itself a crime on top of the crime you conspired to commit.

The reason is the Mafia. They stuck around for such a long time in part because the bosses never committed any crimes themselves, and were often careful to never even directly order anyone else to commit crimes. Nonetheless, they were in charge, and it was quite well understood by the rest of the conspirators exactly what they wanted. But it was effectively impossible for law enforcement to stop them, and that meant there was no way to stop the actual crimes because the supply of lower-level people willing to commit them for sufficient compensation was effectively infinite.

We finally managed to bring them down in part by making conspiracy to commit crime itself be a crime. This was called the Racketeer Influenced and Corrupt Organizations Act or RICO and was passed in the US in 1970. From that point forward, it has been a crime to influence others to commit crimes, and enhanced sentencing is available in cases where this is done as part of an ongoing criminal conspiracy.