And one more reason: deterrence. Supposedly, would-be criminals are deterred by seeing others get jail time. Depending on the crime, that purpose is rarely accomplished as well.
Deterrence exists, but all the research (in the US) suggests it's caused by certainty of punishment, not magnitude of punishment. A two-week stint in jail that's almost guaranteed will do far more to deter criminals than a ten-year stint that almost never happens.
Unfortunately, it's much easier to crank up the penalties and posture as "tough on crime" than it is to make sure police are actually closing cases.
