No, it is not a mere formality. It is necessary for the US to wage a war. The President doesn't have Constitutional authority to conduct a war without a declaration and Congress has no authority to pass a War Powers Act allowing any such thing.
Vietnam was a textbook case of what happens when war is waged without a declaration. The President is hamstrung, and Congress gets to conduct the war by committee. Treason isn't punishable, and Congress becomes a cesspool of treason.
A declaration of war can be issued by Congress against those known to sponsor and commit terrorism, the Taliban government, Hamas, Hezbolla, and every other organization with a name. The US has declared war before on a gang of pirates. Treason in time of war is a hanging offense. With no declaration of war, people like Aldrich Ames go to prison. With a declaration of war, even a sitting Senator would hang for treason, and it would happen quickly by military tribunal.
You should read the US Constitution sometime. It's fascinating stuff, and it's roundly ignored by almost every elected official in the country.
The Civil War, the bloodiest conflict in all American history to date with total battle deaths higher than all other US wars combined, was never a declared war and no formal declaration of war was ever issued. Furthermore, people were imprisoned and even hung for treason during the war, in addition to the same for desertion and spying. You aren't qualified to lecture me about American history, the US Constitution, or the history of warfare. You are caught up in semantics and formalities. Formal war declarations have been ignored far more often than not, and most of our greatest conflicts and military campaigns have taken place without them. Again, treason and its consequences aren't predicated on war conditions. If it were the, aforementioned traitors wouldn't be in prison charged with that offense. The only item lacking is the will to enforce the maximum penalty, not a justification for doing such.