There are historical reasons, like the fact that LE was created to protect a special class), which deeply impact the way LE is trained, which dictates how LE interacts with normal people. There’s what you suggested, that they want great stats, because political careers are made off of stats, and winning high profile cases (there’s a long list of questionable cases where DAs railroaded a person or group just before they ran for governor or congress.
The book “Unfair: The New Science of Criminal Injustice” discusses a lot of the failings of the system, and gives suggestions on how to fix the system without tearing it down. In discussing the failings, he gives a lot of thoughts on why things are as they are, etc. It’s a pretty easy read.
Some people hear "Al Capone was only ever jailed for tax evasion" and think to themselves "that murderer deserved to be in jail, getting him jailed was just even if it wasn't for his biggest crimes"
It can help the FBI nail someone they know is guilty, but is somehow made of teflon so the prosecutor can't prove it in court.
It's also handy for dealing with political opponents. Like imagine Occupy starts up again and gains real traction this time. The sight of its leadership being perp-walked into federal court can be real demoralizing. If the FBI has the ability to arrest them on procedural crimes any time they want, they could break up the entire operation before it poses a threat to the status quo. Do you think COINTELPRO actually went away?
