Satellites move in inertial space, about an Earth that rotates. The motion of the Earth since a fixed epoch (currently noon, January 1, 2000) has to be taken into account for things that depend on the satellite's location over the ground -- some gravitational effects, and the location of radar tracking stations.
We have to account for leap seconds, as that's obviously a component (albeit small) in the equation.
Unless you use a .tle that was computed in UTC when your system is running in GPS time! :-) LOL