Note that the fixed stars define a reference frame with very low absolute rotation (i.e., very low rotation relative to the observable universe) and are used in most practical measurements of absolute rotation and this was done long before our modern understanding of absolute rotation. This is where the adjective "sidereal" (meaning relative to fixed stars) comes from.

Note the solar day = current mean value 86400.002 s (which is the solar noon to solar noon period).

Of course, the standard metric day = 24 h = 86400 s exactly by definition.

The modern international second is defined by an atomic process that according to quantum mechanics is exact and invariant---and we trust quantum mechanics more than we trust any other physics theory.

Yours truly mnemonicks this daily advance by the mnemonic: The stars rise earlier every day.

Sidereal time is used in astronomy since astronomy has to keep track of the motion of the celestial sphere.

In the old days, observatories would have a standard time clock and a sidereal clock superimposed somehow or mounted side-by-side. But nowadays, the computer tells us everything (e.g., Juergen Giesen: sidereal clock; USNO: Compute Local Apparent Sidereal Time).