I have a very similar tank I pulled from a junk yard. It's not pointless at all. In many parts of the USA, diesel stations are much harder to find than gasoline. Besides which, I'd rather go to a station as infrequently as possible, and then get stunned looks from the cashiers when I prepay with cash and they see that it actually takes $200 in fuel when I come back for change.
If you don't drive that much, these also can make a lot of sense as you can fill up whenever the ever-fluctuating gas prices drop, and just skimp by on a few gallons when they're high.
Mine does not have a separate filler cap though - it just fills when you fill the main tank by gravity - there's a ventilation tube that comes from the top center of the tank to a hole drilled behind the regular fuel door. I don't think you should need to use the extra cap to fill yours - just loosen it to allow air to escape while filling the main tank - gravity will take care of the rest.