Coordinates require shared conventions
An angle has meaning only relative to its reference geometry. Equatorial coordinates use right ascension and declination; ecliptic coordinates use a different reference plane rather than merely renaming the same values.
Different coordinate pairs can describe one sightline. Transformations require the correct axes and conventions; a change of representation does not itself move the source.
References: [1]
Catalogue directions and local pointing
Equatorial coordinates organize catalogue directions, while horizontal coordinates specify local azimuth and altitude. The same target normally has different horizontal coordinates at different sites and times.
Pointing from a catalogue can involve observer location, time, aberration, precession and refraction. Horizons consequently distinguishes astrometric, apparent and local-pointing quantities rather than supplying one unconditional pair of numbers.
Frames and epochs travel with the values
Earth’s axis changes slowly and stars have proper motions. Directly subtracting values from different reference conventions can mix frame changes with source motion.
The ICRS supplies a common high-precision system and must be distinguished from traditional epoch-specific equator-and-equinox conventions. Preserving the provider’s frame and epoch is more informative than extra decimal places without those labels.