The dental sibilant is the commonest of the three and the one that carries most of the grammatical load in Sanskrit. It stands in the nominative singular ending of the largest class of nouns, in a whole set of verbal endings, and in the future and aorist formations, so it is met at the end of words constantly. It also opens a substantial family of common vocabulary, including the words for being, for truth, for a companion and for together.
At the end of a word before a pause, however, it is very often not written as this letter at all. Sanskrit converts a final dental sibilant to the visarga, a breath written as two dots standing outside the letters proper, so the sign a reader actually sees at the end of a great many words is the mark rather than the letter. The conversion is conditional: before certain following sounds the sibilant is kept, before others it becomes the visarga, and before others again it assimilates entirely.
That set of rules is among the most frequently applied in the language and among the first a student learns, and it means the relationship between the letter and the sound is looser here than anywhere else in the alphabet. A reader of a continuous text has to run the rules backwards to recover the underlying form of every word that ends in a sibilant.