What made these games different was that the musical themes were significant and well known long before you installed your SoundBlaster. The music was mixed at high intensity out of the box allowing it to influence you, each track tailored to the moment.
This gave the series a leg up in that the music could actually communicate information effectively -- a tense moment, the shifting tide of the battle, the calm after a victory -- whereas other games simply had to put up waveforms that sounded pleasing.
To be fair many games experimented with sound design in this era, but few had such legendary IP to build with. An unfair advantage to say the least. The folks wielding iMUSE clearly knew what they had.
Modern games have similar reactive music systems but I've never heard one I felt was better than X-Wing's. They got it right on the first try.