If you want to sample/monitor activity on the external signals of the MCU, e.g. the signals on the external memory bus, a serial interface would be entirely unuseable. If all you want is to drive a display, then, yes, a serial channel created in firmware might, indeed, be the solution.

If you want to sample the data written to or read from an external memory or peripheral at a specific address, then you might want to latch the data, based on the chip select/nWR/nRD signals. It could be done with a logic analyzer, but ... the LA takes quite a while to set up, while a simple sample/monitor/display circuit could do the job pretty easily and would be widely applicable. Moreover, it would work at logic speeds, rather than poking along at a rate that most MCU's can support.

If you're given to detailed study of your products, you probably incorporate a system bus probe connector for attachment to a logic analyzer or other equipment. That can easily be used in combination with this kind of monitoring circuit.

RE