There are several ways to interpret this question:
- Why does a single operating system require two partitions; more specifically: two volumes?
- Why does that extra partition require a FAT32 filesystem, as opposed to e.g. NTFS or Ext4?
- (implicit question?): What files are on this FAT32 filesystem.
1. The answer to the first is:
It doesn't technically require two partitions, just that the partition with the boot files is FAT32.
Since FAT32 isn't great for running an entire OS the developers are forced to use two filesystems, hence two partitions. I believe Ext4 is common for linux OS partitions.
2. Why FAT32 for the boot files? We speculate the following reasons:
Easy/compact to implement the filesystem driver in a bootloader. Most other filesystems are complexer. Writing a bootloader isn't easy, especially if you need to minimize as much on-chip NAND as possible.
Other filesystems may include (higher) licensing costs or are not properly documented.
The FAT32 filesystem is supported by common operating systems such as Windows, MacOS and Linux. So no need to install special drivers to access the files on the SD card/image.
3. The boot files are like a lobotomized version of GRUB, and I believe they're closed source. It should contain all the files needed to load your kernel of choice, including basic drivers (which may differ from--or be added to--the kernel drivers that are loaded later).
Regarding your analogies:
I've seen this same thing for bootable USB thumb drives as well [...] a
regular hard drive installation of Linux doesn't need to play this
game, does it?
Actually, they did. And do, in some cases. E.g. installing Windows 7 on a BIOS machine will create 2 partitions, a 100MB NTFS partition that contains boot files. It is common for linux machines to have a separate
/boot partition that contains the bootloader (e.g. GRUB) and kernel+driver files. The separation isn't strictly necessary because those bootloaders use the MBR, but using the MBR is 'hackier' than using a proper filesystem. This is why the newer UEFI booting firmware also prefers a separate FAT filesystem for boot files (which is probably what you saw on your thumb drive).