But Kelvin is just applied Celsius. K = C + 273.15
Search for the SI unit of temperature, and you'll find out it's Kelvin (K). It's the one all scientists use, mainly physicists and chemists - due to the fact they work with extremely low temperatures and it is easier to work with such low numbers using it.
Kelvin invented the termodynamic scale that everyone in the scientific field uses currently. All experts begun using this new scale when they realised it was much better for their purposes. For instance, the universal gas and liquid law requires absolute value |º|. If we face a negative number with the Celsius scale we cannot work with it, however the Kelvin scale gives the absolute value and allows for such doing.
On a quick note, Celsius and Kelvin are just like degrees and radians. You are always gonna be told there are 360º in a circle. If you do further mathematics you'll be told there are 2π radians in a circle, and that there is a magic with those when applying them to trigonometry. People tend to use the more comfortable units for their work.