MS-DOS bootable USB flash is still very useful today. It could be used to access a system that doesn't have any OS installed. Or, it's mostly used to flash BIOS or other firmwares from DOS mode.
There is an existing document in the sevenforum.com that gives you step by step instruction on how to create a MS-DOS bootable flash drive using HP USB Disk Storage Format tool. See link here:
However, this method may not work for all the USB flash drvies.
Ex: It does not work for my Kingston 4.0GB USB flash drive
Here is another workaround:
Step1: Download rufus utility, Rufus v 1.1.7 without FreeDOS Support (154KB) from here:
Step2: Download win98_bootdisk.iso image from allbootdisks.com:
Step3: Run rufus_v1.1.7.exe
Step4: click the disk icon to the right of "Create a bootable disk using:", and selected the win98_bootdisk.iso you downloaded in step2.
Step5: click start, once you are finished. You could copy your BIOS upgrade files to this USB flash and run the executables in DOS mode.