Originally it was so each city can form and run it's own militia and the government isn't allowed to stop them forming those militias. A militia is a group of trained soldiers who are not part of the military, in this case with the purpose fighting against the government if the government turns to tyranny.

You were always otherwise allowed them for protection and hunting, but I think around 1900 or so some states began trying to limit gun sales for sporting/hunting purposes, that lead to a reinterpretation that the right to bare arms was actually a right to bare arms first and foremost and not so much a right to form an armed militia.

Ultimately the constitution's meaning is what the courts say it is, but it's really really hard to see the amendment as originally meaning anything but the right to form militias to fight tyranny. You have to be doing some mental gymnastics to think owning guns other than for killing police and soldiers is protected by the second amendment. Of course the government would hate this interpretation and notably they always refer to as the right to bare arms. The arms sellers don't like it either - they want you to buy them for fun and bragging rights, not sniping their other clients.

Like the free speech amendment, there is a goal of standing against a tyrannical government, and an overkill measure so that interpretation can't dilute the right: the government can't act against speech on the off chance it's criticism of the government, and they can't act against gun ownership on the off chance it's for forming a militia. But, again and again the supreme court (notably a part of the government) has interpreted the guns right as a right to own guns.

Strangely the gun nuts with huge weapons caches are probably right on this one.