Qui tam actions serve a special and specific purpose, allowing lawsuits on behalf of the government. Generally speaking, fraud matters end up brought forward the most.
It is important to understand which parties have involvement in each qui tam case due to the fact that one party speaks on behalf of another.
The National Whistleblower Center discusses more about qui tam cases. Each qui tam case involves three parties.
The first party is the person bringing the lawsuit forward, i.e. the relator. They file the lawsuit on behalf of the government, and during the case itself, they act on behalf of the government. Typically this is an employee or even just a citizen who noticed wrongdoing.
The second party is the government. In the lawsuit, they classify as the plaintiff. They receive benefits of the action, but in qui tam cases, they also give part of the reward to the relator if they win the case.
Finally, the third party is the defendant. This is the person, organization, group or entity blamed of wrongdoing, i.e. the entity that supposedly committed fraud or another illegal act against the government. If they lose the case, they will have to pay damages. The damages get divided between the first and second parties.
Because the government cannot have eyes everywhere all at once, they rely on qui tam suits to catch crimes they otherwise may have missed. It is a good way for citizens to protect their own interests, too.