Three Syriac letters can carry a vowel as well as a consonant: alaph, waw and yudh. The device is old and is shared across the Semitic scripts, where such letters are described as mothers of reading, and it is the abjad answer to a problem that a consonantal script otherwise leaves entirely to the reader. Waw carries u and o, yudh carries i and e, and alaph carries a vowel when its own consonant falls silent.
This is a partial solution and it was never meant as a complete one. It marks only long vowels, it does not distinguish the two vowels either letter can stand for, and it leaves the short vowels unwritten. Syriac later built two full pointing systems on top of it, and both were added to a script that already had these three letters doing part of the work.
Waw is the letter where the ambiguity is most often felt, because the difference between u and o carries real weight in Syriac words, and unpointed text does not show it. In practice the reader knows the word. In practice, too, an editor of an unpointed manuscript occasionally does not, and the choice made there is an editorial reading rather than a transcription.
The same three letters do the same work in Hebrew and in Arabic, which is one of the plainest signs that these are one family of scripts and not merely a set of neighbouring ones. What differs is which vowel each letter came to carry, and how much pointing was later built on top.