Absolute zero, at -273.15°C or 0K, represents a state where theoretical molecular motion ceases. While reaching absolute zero is a central concept in thermodynamics, in practice, it's impossible to cool any substance to absolute zero. However, scientists have come extremely close. Advanced techniques in cryogenics and laser cooling have enabled us to achieve temperatures mere billionths of a degree above absolute zero. These extremely low temperatures are crucial for certain quantum experiments and for the study of superfluidity and superconductivity.

While the Kelvin scale is absolute and crucial for many scientific calculations, especially in thermodynamics, the Celsius scale remains practical for a plethora of reasons. Celsius is based on easily observable and replicable reference points (freezing and boiling of water), making it intuitive and easily understood. Most day-to-day phenomena and reactions occur in temperatures that are more relatable in the Celsius scale. Hence, for many experiments and observations, especially those not approaching absolute zero or dealing with extremely high temperatures, the Celsius scale remains more intuitive and convenient.

The shift from Fahrenheit to Celsius in many parts of the world was primarily influenced by the move towards metric standardisation. The Celsius scale, being part of the metric system, integrates seamlessly with other metric measurements, facilitating scientific calculations and experiments. Additionally, its reference points based on the properties of water are easily replicable and understood universally. The global push, especially after the mid-20th century, towards a unified system of measurements for science, trade, and daily life, saw Celsius being favoured over Fahrenheit in most countries. The U.S. remains a notable exception, still predominantly using the Fahrenheit scale.

The body's temperature is variable and is influenced by a range of factors like health, activity levels, and the surrounding environment. While Fahrenheit used the human body temperature as a reference, it's important to note that it was only one of the three reference points he used. Celsius and Kelvin scales chose reference points that are consistent, universally applicable, and easily replicable in a laboratory setting. Using the freezing and boiling points of water as reference makes the Celsius scale more universally standardised and applicable for scientific experiments and daily measurements.

The Fahrenheit scale has smaller increments between its defined freezing and boiling points of water than the Celsius scale. This granularity offers a more detailed representation of temperature changes, especially when slight changes can be meaningful. For instance, in detailed meteorological analyses, a more nuanced temperature representation can provide better insights into weather patterns. Additionally, for everyday non-scientific purposes, especially in regions used to the Fahrenheit scale, this granularity might feel more intuitive and offer a more precise feel of temperature differences without resorting to decimal values.