Most US states have some form of expungement law which allows past criminal records to be cleared. It's mostly intended to help ex-cons with reintegration, because it can be so difficult to get employment (and in many cases voting rights, public services, housing, etc) with a criminal history. Basically criminal records are a major part of the "cycle of criminality" that causes recidivism because former criminals are unable to support themselves by means other than crime (can't get a job, etc), and so expungement policies are expected to have a positive impact on both crime and social welfare. The details all vary from state to state, often there's a limit on how severe of an offense can be expunged, it has to be a certain time in the past, and typically the records are not really "expunged" but removed from public reports---law enforcement and judges can often still see them, as well as federal background checks, and sometimes anyone can ask for a court order to unseal. The expungement itself usually requires a court order, so the court makes a judgment whether or not expungement serves the public interest (i.e. will allow the person to contribute more fully to society). There are a lot of law firms like that that specialize in expungement because there are many people with criminal records and in most cases the process is a pretty straightforward paperwork exercise, so you can make a steady profit stream with a few paralegals preparing form letters.

It becomes a different matter for people who are charged with crimes and otherwise in court, as courts have rules about what criminal history can or cannot be introduced in the court. There's a general principle that a jury shouldn't know about a person's unrelated previous convictions, since it will bias them towards the person being a criminal despite not actually being evidence relevant to the case in question. But I think there are plenty of exceptions, i.e. if the person has been convicted for pretty much the same crime before it can usually be introduced by the prosecution as circumstantial evidence that they apparently have the skills/means/motivation to commit such a crime. But you can't just tell the jury that they have a rap sheet a mile long. I'm not a lawyer and only know so much about this but I believe that usually pretty much the same rules apply to witnesses. You can bring up that a witness has a criminal record if it's somehow directly relevant to their testimony (i.e. they have lied to a court in the past) but you can't just tell the jury that they have a criminal record to try to ruin their reputation. Although it usually fails in practice, the justice system is at least theoretically built around the principle that people can and do change, and so prior criminality shouldn't be held against them if it isn't somehow contributory to the current situation.