It matters because it is the document both sides reach for when something has gone wrong and tempers are short. An SLA written to sound reassuring during the sale is a liability during an outage; one written around what your team can actually deliver on a bad week is a defence.
The common mistake is promising resolution times rather than response times. You control when someone picks up the ticket. You do not control when a vendor releases a fix, when a client's staff become available, or when a failed drive arrives. Committing to an outcome that depends on other people is how an SLA becomes unmeetable in exactly the situations it was written for.