The stroke drawn above a letter to mark it as a number does a second and older job. Written over a contracted word it signals an abbreviation, and the abbreviations it governs are not chosen for convenience.
The practice comes from Greek, where a small set of sacred words, God, Lord, Jesus, Christ, Spirit, Saviour, Mother, were written contracted to their first and last letters under a stroke rather than spelled out. Scholars call them the nomina sacra. The convention is older than any surviving Slavonic book and was already universal in Christian Greek manuscripts, so the Slavonic translators inherited it along with everything else they were copying, and applied it to the Slavonic equivalents of the same words.
The effect on a page is immediate and visual. The sacred names are the shortest words in a line and they are the ones carrying a mark, so the eye finds them before the mind reads them. A word treated this way is not being shortened to save parchment; it is being set apart, in the same spirit that governs how such words are spoken.
For a modern reader the convention is the first real obstacle in a Slavonic manuscript, because the contracted forms are not guessable without knowing the list. For the reader the books were made for it was the opposite: the contraction was the most legible thing on the page, and an expanded spelling of a holy name would have looked wrong.