It's just designed with a slightly different set of assumptions.

Instead of water freezing and boiling 100° apart, it's 180° in fahrenheit. That makes it so that they're on the opposite sides of a temperature gauge, and a degree of rotation of the gauge matches a degree of temperature.
Instead of zero being the freezing point of water under specific conditions, it's a brine solution whose temperature will stabilize in a way that's useful for using as a calibration point.

Stripped of its context, it's odd. But it's not irrational, just no longer consensus as the standard, and as such deprecated.

Fahrenheit makes sense for humans. Most of your day to day climates are in the 0 to 100 scale, and every 10 degrees is a noticeable level change.

• 100 super hot day, approaching unsafe without counter measures
• 90 really hot day, slightly annoying and should take precautions
• 80, hot day, more annoying than anything
• 70, beautiful day, enjoy it
• 60 not to bad, if it's windy you could be slightly on the cold side
• 50 long sleeves or maybe a hoodie
• 40 definitely a jacket, and hat
• 30 full on coat, scarfe, and hat
• 20 multiple layers of out for a while, maybe double pants
• 10 annoyingly cold, need to start thinking about the safety precautions
• 0 and below, temperature now measured in hold long you can be outside before danger

Celsius makes sense for science stuff because it's derived from science stuff, so things like calories and energy work with it. But it doesn't really apply to everyday life as well. So it actually makes sense to use both units for the things the are good at.

Never. They use the same spacing between degrees. The Kelvin scale was derived from the Celsius scale, just placing the 0° at absolute zero rather than at the freezing point of water.