The vocalic l is the rarest of the four liquid vowels and one of the rarest letters in the whole table. In classical Sanskrit it is effectively confined to the forms of a single verbal root and its derivatives, which means a reader can work through a substantial library without meeting it outside a letter chart. Its dependent form, like the other liquid vowels, is written beneath the consonant.
The Indian analysis nonetheless treats it as a full member of the vowel system, on the same footing as a, i and u. The reasoning is structural. The phoneticians recognised that r and l behave alike, that both can carry a syllable, and that a description of the sound inventory admitting one and excluding the other would be inconsistent with itself. Consistency of that kind mattered more to them than the count of words affected, and the table records the decision.
It is worth noticing what that implies about the object being transmitted. A syllabary containing a letter for a sound found in one root is not a practical inventory of what has to be written. It is a model of the sound system, in which the letters are the elements of an analysis. Siddham inherited the model whole, and the traditions that carried the script east preserved it in that form rather than reducing it to whatever their own use required.