To the uninitiated about to be owner, where/what is this joint and why does it need lubricating?
More properly, it's a seam rather than a joint-where the panels join. You will see the running board, curved panel where the bonnet closes, and also the bulkhead, plus where the rubber tread strips meet the running board if you have extra tread rubbers
They can all be water traps, so there is a need to put something in there to try and prevent either rust or salts reaction between the different metals
HTH
G
PS don't forget to get all mud, dirt etc off the UNDERSIDE of the running boards when you wash the car
