Reading the table
An uptime percentage is a budget, not a promise. 99.9% does not mean "almost never down", it means you may be down for 43m 12s a month without breaching. Three nines sounds close to four, but four nines cuts the monthly allowance to 4m 19s, which is less time than most teams take to notice an incident, let alone fix one.
That gap is the practical argument for monitoring. At 99.99% you have roughly four minutes a month, so detection time is most of your budget. A check that runs every five minutes can burn the entire allowance before it fires once.
99% uptime allows 14m 24s of downtime per day, 7h 12m per month, and 3d 15h per year. Common entry-level commitment.
99.9% uptime allows 1m 26s of downtime per day, 43m 12s per month, and 8h 45m per year. The standard SaaS SLA.
99.99% uptime allows 9s of downtime per day, 4m 19s per month, and 52m 34s per year. Enterprise commitment.