They are libraries that are not designed for singing, but for use with sequencers, so there is no correspondence between note pitches and their position on the keyboard or, even worse, on the staff. The principle is that organizing certain phonemes, although they do not necessarily form meaningful words, is more realistic than simple vocalizations with the classic ah, eh, oh, etc...; it gives an appearance of text even if you cannot understand which words they are. This works especially in an orchestral context and in epic-style tracks.
I will cite only one example: the soundtrack of the video game Skyrim (we are talking about one of the most successful video games of all time) makes extensive use of samples from the 8Dio Requiem Pro library; I can distinguish the words because I know the library, but they are just words placed next to each other without a complete meaning: Crucifixus, Dominum, Morituri, Ex crux, Incarnatus, Apocalypse, etc... for an Ave Maria, it is not suitable.
Regarding whether or not to recommend purchasing them, it depends: if you have the budget available and if you like tinkering with this stuff, it certainly gives more satisfaction than simple vocalizations, but as with all libraries, you need to know how to use them to optimize results and have certain system requirements because they are demanding in terms of RAM.
I am happy with Soundiron libraries: Venus, Mars, and Mercury, but there are others, like Voxos, which I don't like very much; in terms of sampling quality they are better than EW, but only EW has a true wordbuilder.