Over flow bottle ???
Other than
@Dana67Dart, I can't believe there are so many goofy answers from smart people on this thread. It's a very simple concept: When liquid in the cooling system expands, it comes under pressure. When that pressure goes higher than the rating of the radiator cap, it needs someplace to go. You could just let it dump out under the car, but then you lost some coolant permanently. So you hook it up to a bottle, which catches the overflow. And because the tube's entry into the bottle is submerged in the overflow fluid, it naturally sucks the fluid back into the radiator when the system cools. Repeats every time the car is run up to temp.
I'm not so worried about tainting race tracks (I don't race) or the cost of a bit of fluid. But this simple system does ensure that my radiator is always filled to the top, with zero air in there. I may be wrong, but I suspect that having zero air in the radiator is helpful in preventing corrosion. My 72 Swinger seems to have its original 52 year old radiator in it working fine (I've only had the car for 27 years, so I'm not positive).
But for this to work, you definitely need a little hole in the top of the bottle so that air can come and go as fluid comes and goes. I have that same exact cheapo bottle on my car and it didn't come vented, so I drilled a 1/16" hole in the top and it works great. It was a great replacement for my old solution, which was just a plastic bottle with a tube going to the bottom.