The comparison with an early PC is not imaginative.
Think of the original Sinclair ZX81 with 1K of RAM and 8K ROM. Swap out the Z80 for a 6502 to get approximately the same register count. Without a screen to worry about (it took up too much RAM + CPU time) and you could do a lot, e.g. 1K chess or, as the adverts said, run a nucular power station with it. You could also service I/O with hardware interrupts.
Think of the original Sinclair ZX81 with 1K of RAM and 8K ROM. Swap out the Z80 for a 6502 to get approximately the same register count. Without a screen to worry about (it took up too much RAM + CPU time) and you could do a lot, e.g. 1K chess or, as the adverts said, run a nucular power station with it. You could also service I/O with hardware interrupts.