Suicide is a choice towards ending one's "mortal" existence. Only that individual can know the circumstances of their intent, even when a message or "evidence" is left behind. It obviously changes nothing, pertaining to the deceased's reason, for anyone else to know. When others say "If we can find out why they did what they did, "maybe" it can help someone else." this is simply a justification for pursuing such information.
When we learn to be primarily "listeners" vs. self-focused "talkers, people who are considering ending their life (and at times others, in conjunction with), as well as all people, will have an increased chance of not making decisions for wrong or incomplete reasons.