There are numerous threads on supertweeters. The most common explanation for their demonstrable effectiveness is the rendition of upper harmonics above the audible range. Specifically they work well with DSD and high resolution recordings. The aim is not really to improve treble since they mostly work beyond the audible range and any normal tweeter will easily cover the upper limit of most people‘s hearing ability.
One other hint: keep moving them forward and backward in very small, i.e. 1mm increments from the seating position. The phase allignment with the tweeter is very important, i.e. in opposite phase they cancel each other out in the audible spectrum. Equally, their sensitivity should ideally not differ too much from the speaker; a limited amount of adjustment here can be achieved by toe in.