An abjad records consonants and leaves the vowels to the reader, and Syriac has two separate answers to the problem of writing them down anyway. The eastern hand marks vowels with arrangements of dots placed above and below the line. The western hand writes small Greek vowel letters over the consonants, a borrowing that records how closely Syriac scholarship stood to Greek learning in the centuries when the systems were fixed. Both are additions to a script that had worked without them for a long time, and ordinary running text still leaves them off and expects the reader to supply what is missing.
Alaph is where that problem is sharpest. Because its own consonant is so often silent, a word beginning with alaph begins, to the ear, with a vowel, and the letter is holding a place for a sound it does not itself write. Nothing on the page distinguishes one such vowel from another unless the pointing is present. A reader who knows the language does not notice the gap; a reader learning it notices very little else.
Alaph also carries the numeral value one. Syriac letters double as figures, so alaph opens the units in the same way it opens the alphabet, and a scribe numbering the leaves and quires of a manuscript begins with it. The two jobs sit on one sign without interfering, because a figure is marked as a figure by the way it is written into the line rather than by any change to the letter.