UART boot mode allows loading boot binaries into memory that is already initialized by ROM. It is useful for being the simplest boot-mode that does not require any other modules except the UART. The DM firmware can be loaded, and this will take over and load the rest of the boot binaries required. The down-side to UART boot-mode is the speed: it will have a default speed set by ROM, and this can affect loading large applications. It is a great initial boot mode for initial checkout, but in later stages it is preferred to use other boot-modes.