The main reason for charging conspiracy rather than actually selling, possessing or using drugs, however, is that the conspiracy laws now allow imposition of those dreadful mandatory minimum sentences on users and the smallest of sellers. By definition, knowingly buying an illegal substance has always been a conspiracy between buyer and seller. But a buyer is now presumed to know that the seller is a dealer, and can be charged with conspiracy for all the drug sold by that dealer, and be subject to the same penalties. That goes for all of a dealer's customers. Since judges no longer have discretion in drug sentences, the only way out is to cop a plea with the prosecution by snitching on others.
Dealers routinely do that, and usually end up with a fine and short sentence, often just parole.
The customers receive the mandatory minimums though, unless they snitch on others. Many do just that. And truth is not a prerequisite. He is caught dealing and drugs are found hidden in her home or car, and she is presumed to have been aware of his activities. The same applies to all of his customers.