What is Download Mode on Android?
Download Mode on Android is for optimizing device. It will help you to flash radio, firmware and any other related files by connecting to the computer.
In my opinion, Download Mode is a boot mode over the generic boot for many Android devices, it have a bootloader code, it should be can be found in the boot partition. For example, this is just one small example, Odin is commonly used on Samsung devices to flash files, those devices needs to be in download mode prior to flashing.
So propularly speaking, Download Mode on Android is just like a gateway for your devices to flash files directly from computer.