What the letter does legally
It puts the other side formally in default — which starts default interest running and strengthens every step that follows. Where the contract prescribes a notice procedure before termination or before a claim, the letter is also the step that unlocks those rights, and skipping it can invalidate what comes next.
What it does practically
It signals that you are organised and willing to escalate, while giving the other side a defined and face-saving way to comply. Many disputes end here precisely because both effects land at once — the debtor sees a real timetable, and is offered a way out that does not require admitting anything in public.
What makes it effective
The right addressee, a clear statement of what is demanded and on what basis, a defined period to comply, and delivery by the mode the contract or law requires — with proof it arrived. A letter that misses the contractual notice clause can fail entirely, so checking that clause comes before drafting a word.
When to skip it
When warning would let assets be moved, evidence destroyed or a limitation period expire. In those situations the first step is the filing, sometimes with an application for interim measures. That is a judgement about the specific counterparty, not a general preference for aggression.
Decide the follow-through before sending
A demand with no plan behind it teaches the other side that deadlines from you are negotiable. Agree in advance what happens if the date passes in silence — a final notice, mediation, or proceedings — so that their silence never becomes your drift.