“Mā lā yalzam” means something not independently required by the basic law of metre or rhyme, not something pointless or irrational. The poet could omit the extra condition but elects to sustain it for greater sonic patterning.
The commitment may involve one earlier letter or several preceding sounds. In every case the additional restriction is self-imposed, rather than the necessary technical condition of rhyme.
Such consistency can enrich cadence, provided the chosen words remain meaningful and fluent instead of being dictated solely by the imposed pattern.