Old Church Slavonic had two nasal vowels, a front one and a back one, and gave each a letter. Almost every Slavic language since has lost them. Polish is the principal exception and still writes two nasal vowels today.
That combination of facts does a particular kind of work. A liturgical language that nobody speaks natively is always open to the suspicion of being a construction, assembled by scholars out of several dialects or out of theory, and the suspicion is not unreasonable given how many liturgical languages are exactly that. The nasal vowels are the answer to it.
Nobody constructing a written language in the ninth century would invent two vowels their own speech did not contain, give each a letter, and then write them consistently in the right places across a large body of translated text. The letters are used systematically, in positions that correspond regularly to nasal vowels reconstructed independently from the comparative evidence of the modern languages, and they line up with what Polish still has. The only economical explanation is that the people writing them were writing what they said.
That makes Old Church Slavonic a record of a real spoken tongue at a datable moment, which is why it matters far beyond liturgy. It is the earliest substantial written Slavic, and the reconstruction of Slavic prehistory rests on it heavily.
The yus letters are the load bearing part of that argument, and it is worth knowing which small marks on a manuscript page are carrying the weight.