This area of law is very tricky. As I've stated above, the technical (i.e., legal) interpretation can be quite different from how a layperson would understand the same words.
The basic idea here is that an authority (e.g., Congress) that has been given a specific power (e.g., declaring war) is obligated to exercise that power where appropriate.
Do note the specific phrasing above: if there is a situation in which exercising that power is the appropriate course of action, then that power must be exercised. The authority cannot choose not to exercise the power in such a situation.
So, Congress doesn't have to declare war for the sake of declaring war. However, if there is an existential military threat to the USA (e.g., aggression by the Japanese Empire in WW2) you could argue that Congress is obligated to declare war, as this would be an appropriate situation to exercise that power.
Of course, the above explanation is very simplistic. There can be specific wordings that clarify how much discretion that authority has; there may be case law pertinent to that area; and whether a given situation mandates "appropriate" exercise of a given power is never a black-and-white decision.