First-touch credits a customer to the channel that first brought them in — the ad clicked months before the call. It answers 'what starts relationships?', which is the question that matters when you're deciding where discovery budget goes.
It matters intensely in one more case: paying anyone (partner, rep, agency) based on 'who sourced this customer'. Then first-touch isn't philosophy, it's the ledger — and it must be captured automatically at the first form-fill, because human memory reconstructs it badly.
The honest caveat: every attribution model is a simplification. First-touch under-credits the follow-up that closed; last-touch under-credits the discovery. Pick per question, apply consistently, and never switch models mid-argument.