The b: drive was usually used to speed up a disk copy, especially to multiple copies, it was way faster than swapping out the disks. Remember that sometimes the whole contents of a disk didn’t fit in available memory. There was a lot of disk swapping.

You could also just keep the msdos disk in a: and execute software from b: Also sometimes a command required reading from the msdos disk. It was handy to have the boot disk always available.

I don’t recall using any software that specifically required two drives, but no doubt it existed.