Malar is the measure of the monsoon, associated across northern India with cloud, with the breaking of the heat and with reunion after long absence, and the section carries a var.
The Granth uses the association and turns it. The obvious reading of a rain measure in a devotional volume is relief: the drought ends, the beloved returns, the ground drinks. What the compositions more often do is keep the attention on the drought. The rain that ends it is also the measure of how parched the ground was, and a soul not watered by the divine name stays cracked through the wettest month there is.
The convention being worked is old and was not invented here. In the poetry the Gurus inherited, the monsoon is the season of return, when travellers come home and separation ends, which is exactly why its arrival sharpens the pain of anyone whose separation does not end. That is the note the Granth reaches for.
Alongside the imagery, the practical association is real: in an agricultural society the monsoon is the difference between a harvest and a famine, and a measure sung at its coming carries a weight that has nothing to do with pastoral sentiment. The rain is not a symbol of grace, it is the thing everyone was waiting for, and the compositions borrow that urgency rather than manufacturing it.