Synchronisation makes several screens play in step, so content that spans them stays aligned frame by frame. Without it, players drift: each starts its loop a fraction later than the last, and within an hour a sequence designed to sweep across five screens is visibly ragged.
It is what makes a row of displays read as one canvas rather than five televisions. Retail window arrays, ribbon displays above a counter, and multi-screen brand moments all depend on it, and the effect collapses entirely if the timing slips.
The caveat is that synchronisation is a spectrum, and vendors rarely say where they sit on it. Loop-level sync (every player restarts its playlist together) is easy and enough for most cases. Frame-level sync, where a moving object crosses a bezel without tearing, is a much harder problem needing a genuine timing signal between players. Ask which one is on offer, and ask to see it on motion, not on a static image.
Also ask what happens when one player reboots mid-loop. A well-behaved system brings it back in step at the next cycle boundary; a poor one leaves it permanently offset until someone notices.