A demand letter is the document that converts your crash into a claim: it lays out why the other party is liable, what the injuries and treatment have been, itemizes economic and non-economic damages, and states the amount demanded — usually with medical records and bills attached.
It matters because it frames the entire negotiation. A demand assembled after treatment has stabilized, with complete records and a coherent liability story, is hard to lowball; a premature or thin one anchors the case low.
Timing is strategic: send it too early and later treatment is left out; too late and you bump into the statute of limitations. This is also the document where an attorney's drafting experience most visibly changes outcomes.