I did but all I get is of how to add support for that, not about why it could be neccessary. I assume this is for A/B partitioning and stuff? Spare partitions?
I found this on a wiki page:
Amongst the standard sysupgrade firmware for the barebones Unifi 6 LR router that comes with the stock Uboot, there is also a “uboot mod” version that takes advantage of the dual-boot and ARM Trustzone but they are a little more complicated to flash. This version works by flashing the preloader, uboot and recovery partitions in addition to the rootfs/OS partition. There isn't much benefit other than having slightly more storage space, debugging crashes and making use of fallback dual-boot partition. The instructions for flashing this version are as they follow: