The rk3588 soc has a fixed boot order which is spi, emmc, microsd, other I think. So if there is a boot loader found on spi it will ignore other options and try to boot from that.
While Armbian uboot binaries on spi are instructed to check if there is a microsd card inserted and boot from that, vendors can (ab)use this circumstance to make it harder for other operation systems being used because tools like rkdevtool are necessary to get rid of it. If the spi is pre-flashed with crap, I guess that's why the step is necessary.