Almost every script descended from the Northwest Semitic alphabet is an abjad. It writes consonants, and the reader supplies the vowels from knowing the language. Hebrew, Syriac and Arabic all work this way, and all three later developed elaborate systems of points and marks to write the vowels in when it mattered, for scripture, for teaching, for poetry.
Mandaic is the exception in its immediate family. Three of its letters, halqa, ushenna and aksa, are full vowel letters for a, u and i. They are not points, they are not optional marks, and they are not consonants doing double duty; they sit in the line with everything else. A Mandaic word is written with its vowels in it.
Why this happened is not something the atlas can settle, and speculation about it is easy to find and hard to check. What can be said is what it does. It makes the script easier to read without training than its neighbours, it removes the ambiguity that unpointed Hebrew and Syriac carry, and it means that a Mandaic manuscript preserves the vowels of the language as its scribes pronounced them, which is evidence about the language that unpointed scripts simply do not supply.
The vocalization mark that can distinguish vowel quality is a refinement on top of that, used mainly in teaching. It is not doing the job that Hebrew and Syriac pointing does, because that job is already done by the letters.