" If all it did was choose between 2 batteries then a simple 1/2 selector switch could do that without electrical
stuff to complicate things "
Not really. Consider that you want maximum power to the starter. That means ideally you would want the cable from the battery(s) to the starter relay being as short as possible. If you do that with an A/B switch, the switch might need to be located 4? 8? 10? feet away from the actually battery cables
, creating (both directions add) possibly a 12-20' extra power drop in the cable.
By using a relay, you can put the A/B switch where it is convenient to the electrical
panel, and still keep the actual power cables
really short (and cheaper and more efficient).
Sure, it violates KISS. And that unit looks like it has been rather neglected and possibly corroded. But the idea is solid, and a common way to avoid line power losses.