If eMMC has a system with a u-boot that supports starting from USB, I can easily start any additional systems without any additional configuration, just plug in the USB and turn on the power. For example, I have Armbian installed in eMMC for main use, but I can connect USB with LE to start it immediately (I don't change anything in the existing system in eMMC). Same with the SD card. I don't need anything to pull out and change, just plug in the USB and immediately get a fully working USB system (including the kernel and everything else). It is much easier to use USB, I can write it on absolutely any PC, all of them have USB. Card reader-rarely available on a PC. The same goes for USB distribution, it's much easier to buy and use anywhere than an SD card. By the way, USB 3.0 speeds are now much higher than any SD card. Yes, initially I may need an SD card, for a one-time write to eMMC of a new system with u-boot with support for starting from USB. Or if there is Android in eMMC, I can replace u-boot without an SD card, using only USB.