I'll start with your second question first. No, Rome did not need every available soldier in the defense of Rome. Immediately after Cannae, two legions were formed from the survivors under command of the consul Varro. (Varro received a hero's welcome in Rome for organising the survivors and not giving up.) These two legions, together with some marines and a couple of newly raised urban legions served as an emergency blocking force should Hannibal decide to march on Rome (Hannibal didn't).

Meanwhile at Rome four new legions were formed in all haste. They enlisted citizens as young as 17, prisoners who were released for this purpose, and several thousand slaves who were purchased and freed so they could serve in the army. These four legions would form the new army for southern Italy. The Second Punic War was not limited to fighting Hannibal though. For much of the war Rome had armies of varying sizes in northern Italy (fighting Gauls), Spain (attacking Hannibal's home base), Illyria/Greece (fighting Philip V of Macedon, who became an ally of Hannibal after Cannae) and Sicily.

Rome had taken control over Sicily from the Carthaginians during the First Punic War (about 25 years earlier). At the start of the Second Punic War Rome sent two legions to Sicily. After Cannae, the two legions of survivors were sent to Sicily and the Senate refused to allow them to return Italy. The legions in Sicily had a few purposes:

• To defend against any Carthaginian invasion (they did land a force of 30.000);
• To ensure (by force if need be) the continued loyalty of Sicilian cities;
• To secure taxes (grain) which were important to feed the Roman armies.

This last point was especially important. Sicily and Sardinia were the main sources of food for the Romans, certainly with the Italian countryside ravaged by war.

So to answer your first question: being sent to Sicily was a form of punishment because it really was a side-show with little opportunity for glory or loot. In Italy the legions were defending Rome and fighting a legendary enemy. In Spain the legions were attacking the enemy's home base, with lots of opportunity for glory and looting. And in Sicily? The legions were propping up pro-Roman politicians and securing taxes while keeping a small Carthaginian army busy. The most notable event in Sicily was the siege of Syracuse, where the Romans had demanded the extradition of pro-Carthaginian politicians and the restoration of pro-Roman government.

The Sicilian legions from Cannae would eventually form the core of Publius Cornelius Scipio's invasion army for northern Africa, and they would defeat Hannibal at the battle of Zama, so in the end they did redeem themselves.