Wires are like water pipes for electricity. The bigger the pipe, the higher volume (amperage) you can transport. The more volume you try to transport through any one pipe, the more the pressure (voltage) drops in that pipe. If you have a big wire and only transport a little bit of current, you will have a very small voltage drop. If instead, you have a small wire and try to transport a lot of current, you will have a large voltage drop. This also shows up in the form of heat - transport too much current through a wire and it will heat up to the point it melts.
So what does all this have to do with headlights? Well, because there is only one wire feeding the fuse box, that wire has to carry all the current for the ignition, the radio, the heater fan, the windshield wipers, the gauges, and the headlights. To make matters worse, the connectors have corroded over the years, adding resistance, and the headlight on/off switch has plenty of resistance itself. By the time the voltage gets to the headlights, it can be significantly lower than what it is at the battery.
Now, the absolute best way to fix the problem is to route wires and relays straight to the headlights. There are three things I don't like about this: one, it costs a lot, two, it is often overkill, and three, it is hard to maintain a stock look. The addition of this kit allows you to pull the headlight voltage from an alternate source (such as our Accessory Fuse Panel), distributing the total electrical load over more wires. This kit is a direct plug-in to your floor mounted High/Low beam selector switch and allows the battery voltage to bypass the factory wiring to the stock fuse box, the 40+ year old corroded fuse box, and the worn-out headlight switch. This will almost certainly brighten your headlights to a much more reasonable level.