The Samaritan alphabet is not used for one language. Beside Samaritan Hebrew, the language of the Pentateuch, the community writes Samaritan Aramaic, and a large part of its literature is in it. Aramaic was the everyday language of the region for centuries, and the Samaritan books that grew up in that period are in the Samaritan dialect of it, written in these letters.
Three bodies of work stand out. There is a Samaritan Targum, an Aramaic rendering of the Pentateuch running alongside the Hebrew text, which is one of the principal witnesses to how the community understood its own scripture. There is a substantial exegetical and homiletic literature, the best known of which is the collection associated with the name Marqe, a foundational Samaritan author whose work is central to the tradition. And there is the liturgy, a body of hymns and prayers assembled over many centuries, in which Aramaic and Hebrew material sits side by side.
Later, as Arabic became the spoken language, the community wrote in Arabic as well and produced translations and commentaries in it. The alphabet went on being used across that shift.
The point is that this script belongs to a people rather than to a language. It has carried three languages for one community, which is what an alphabet is for and a good deal more than most scripts of comparable age are still doing.