In theory, concurrent sentences are an acknowledgement that it is not fair to give multiple punishments for the same crime. However, it is often desirable to charge someone with multiple offences fir the same crime, as they might be found innocent of the more serious offense (or have some of the convuctions overturned on appeal).
For example, in the case of a homicide, you often see the defendent charged with both murder and manslaughter for the same act. In such a case the defendent would likely get a concurrent sentence because they were only convicted of a single act.
In many cases, the line between multiple convictions being a single “act” is blurry, the judge can exersise discretion.