Warming up is treated as a non-negotiable step, largely on the grounds that it prevents injury. The performance case is stronger than the injury case.
What raising temperature actually does
Light activity increases muscle temperature, which speeds the chemical reactions involved in contraction and reduces the internal resistance of the tissue.
Blood flow redistributes toward working muscle, and the oxygen delivery system moves closer to the state the coming effort will require.
These changes are measurable and reasonably quick, which is why a short period of easy movement produces most of the available effect. Extending the period well beyond that adds little beyond fatigue.
The nervous system component matters
Repeating a movement at low intensity before performing it hard improves coordination of the muscles involved and reduces early hesitancy.
This is why rehearsing the specific movement pattern outperforms generic activity for anything technically demanding.
Lifters working up through progressively heavier sets are doing this deliberately, and the practice is about preparation rather than about accumulating fatigue.
The injury evidence is mixed
Studies that tested general warm-ups and static stretching against injury rates have not produced the clear protective effect the advice implies.
Structured programs combining strength work, balance and landing technique have shown more promise in specific sports, but those are training programs rather than a brief preliminary.
The honest summary is that warm-ups help performance clearly and help injury rates less clearly than they are usually credited with. Absence of strong evidence is not evidence that they do nothing, and the performance case alone justifies the practice.
Static stretching has a timing problem
Holding a stretch for a prolonged period before an activity requiring force or speed can temporarily reduce power output.
The effect is modest and mostly relevant to performance rather than safety, but it is the reason many programs moved stretching to after the session.
Movement through a range of motion, rather than holding a position at its end, has largely replaced it in the preparatory phase.
What a proportionate warm-up looks like
Effort should match what is coming. A recreational walk needs almost nothing, while sprinting or heavy lifting warrants a genuine progression.
Longer is not automatically better, since a warm-up that produces meaningful fatigue has begun to work against the session it was meant to support.
Anyone returning to exercise after injury, surgery or a long absence should get individual guidance from a clinician or qualified trainer rather than following a general template.