Electrically, they're identical.
The starter motor takes a lot of juice to run. Rather than having an enormous set of contacts in the ignition switch, and the giant wiring to back it up, the ignition switch triggers the solenoid, which does the heavy lifting. The "only" thing the ignition switch and wiring need to run is the solenoid, so they don't have to be so beefy.
The solenoid is really just a relay on steroids. It's an electomagnet, and some enormous electrical contacts. When the electromagnet is energized, it closes the contacts.
When things heat up, two things happen:
1) magnets get weaker
2) things expand, and tolerances change
So you have a double whammy...the moving bits start to bind because of the heat, and the magnet is weaker, so it doesn't get as big of a push from the electromagnet. Plus, if you're running headers, chances are they are heating things up down there more than stock.
Problem is...add enough heat, and the ignition switch and wiring aren't beefy enough any more. Remember that the stock wiring has some detours along the way. The +12 goes through the bulkhead connector, then the fuse block, to the ignition switch, to the neutral safety switch, back through the bulkhead connector, and then over to the starter. Oh and remember to add 30+ years of corrosion on each of those connections along the way.
The result is a) less than +12V, b) lower current than stock, or c) both.
The external solenoid can be mounted so the heat doesn't affect it, and that can in turn feed a nice solid +12 to the solenoid mounted on the starter to power through the heat problems down there.