The Tibetan letters do duty as an ordering series in the way that a, b, c do in English, and the place a reader is most likely to meet that use is on the spine of a book.
The two great canonical collections of Tibetan Buddhism, the Kangyur of translated word and the Tengyur of translated treatise, run to more than three hundred volumes between them in the printed editions. Their volumes are not numbered with digits. They are lettered, ka first, then kha, ga, nga and onward through the thirty, and when the thirty are exhausted the series continues with doubled forms. A citation of a canonical text therefore reads as a letter and a folio rather than as a volume number and a page, and a scholar who wants to follow a reference has to know the order of the alphabet as an ordinal sequence and not merely as a set of shapes.
The same habit runs through ordinary writing. Sections of a treatise, the branches of an outline, the items of a list and the parts of a woodblock set are all marked with ka, kha, ga and the rest. Tibetan does have its own digits and uses them freely for dates and quantities, so this is a choice rather than a shortage: the letters carry sequence and the digits carry number, and the two systems sit side by side without competing.
It is a small thing that changes how the alphabet is held in the mind. Ka is not only a sound. It is also the word for first.