Boot will start floppy images or hard disk images independent of the operating system emulation offered by DOSBox. This will allow you to play booter floppies or boot other operating systems inside DOSBox.
If the target emulated system is PCjr (machine=pcjr) the boot command can be used to load PCjr cartridges (.jrc).

Something worth mentioning is that the bootable images must be mounted before. You can't just start Dosbox and do
z:>boot c:\dos\images\dos1.img c:\dos\images\dos2.img -l a

You have to do:
z:>mount c c:\dos\images
z:>boot dos1.img dos2.img -l a

Same for PCjr.
This always makes me stumble when I once in six months want to boot multiple images 😀