Despite its awesome cool factor, it could be used for a variety of information-gathering technologies. Extrapolating from how humans use lasers, it is not a far reach to think the Borg could also use such technologies for similar reasons.
Wikipedia reminds us:
The basic technology is called LIDAR (Light Detection And
Ranging, also LADAR) is an optical remote sensing technology that can
measure the distance to, or other properties of a target by
illuminating the target with light, often using pulses from a laser.
LIDAR technology has application in geomatics, archaeology, geography,
geology, geomorphology, seismology, forestry, remote sensing and
atmospheric physics,1 as well as in airborne laser swath mapping
(ALSM), laser altimetry and LIDAR contour mapping.
Coupled with the Borg sophisticated computer technology:
the laser emitter could be connected with a variety of information gathering sciences, allowing them to learn a vast array of information about their enemies.
The most likely use of that laser is to detect range, biological information, spectroscopic analysis (similar to how bomb detection lasers function) matter density, armor density and composition to name just a few.
From a production standpoint
To be fair, it is unlikely but not impossible anyone would have considered this when they were adding the visual components to the Borg costuming and just considered it an imposing lighting effect, especially through the stage smoke.
How fortuitous that technology has progressed enough so a purpose could be theoretically assigned to the effect and have it be a reasonable one. Star Trek again, in a strange way, predicts the future.