Those numbers are easy to remember with what they mean. 0, 10, 20, 30 is an easy sequence. So is "freezing", "cold", "warm", "hot".
It's an easy guide to learning Celsius in a little more native way. The translation to Fahrenheit is provided, but not intended to be explicitly memorized. You can always do the C * 2 - 10% + 32 = F mentally if you really need to.
I'm not American so I won't be converting to F, but I somewhat agree with the person I responded to that 30°C / 86°F is not really hot... unless it also happens to be humid, in which case, that temperature can be quite uncomfortable.