In general, 'degrees' are found in units which are mainly
arbitrary. Like, degrees Fahrenheit and Celsius, which
are both arbitrary in the sense that their origin is
chosen in an arbitrary way, and they are meant to be
convenient instead of absolute. The same goes for angle
degrees, since that also divides the circle into 360
degrees arbitrarily.
When the units are absolute, or measured directly with
respect to something else, one drops 'degrees' from the
units. Like, there is no 'degrees' in radians, which is
the 'natural' way of measuring angles. (One can
appreciate the 'natural'ness of radians after one sees
how sines and cosines work, and some series expansion
of those, and their relations with the exponential
function, and complex numbers...).
It is the same way for Kelvin. Temperature is defined
in terms of the average energy of particles in a system,
and Kelvin is directly proportional to that -- the zero
in the Kelvin scale corresponds to absolute zero, and
not any arbitrary temperature, and Kelvin is the
'natural' unit to measure temperature.
However, let me also note that degrees are still used
with Kelvin in some sources, even textbooks, although
it is generally agreed that they should not.
Answered by: Yasar Safkan, Ph.D. M.I.T., Software Engineer, Istanbul, Turkey