Not a stupid question at all! Normally, something (like a caravan) attached to a towbar via the towball. The ball and the hitch become a ball-and-socket joint like your hip. Most of the weight of the caravan is taken on its wheels, so the load on the towball is really just the "drag" of the caravan, plus a bit of downward load because of the noseweight on the tow hitch. When you go round a bend, there's also a bit of side load.
When you put one of these bike carriers on, there's no "drag" load pulling backwards on the towball, like a trailer, but the bike rack has no wheels so it's like a big crowbar trying to wrench the towball off the tow hitch. The longer the bike rack, and the heavier the bikes, the bigger that leverage is. I don't know if that makes any sense? It's hard to explain without pictures! Please don't panic over it - the towball and the bike rack manufacturers will have done loads of testing on their products, but I've always felt that the really long ones are a bit "cruel" to the back half of the vehicle (and the towball and it's mounting bolts)!
Yes, the bikes have individual slots for their wheels, but the slots are not two "half handlebars" width apart, if that makes any sense? You're best putting the first bike on facing (say) left, and the second one facing right, and the third facing left...and so on, so that the handlebars aren't next to each other. The same is true of pedals. This works well when all the bikes are the same size and shape, but we've found that it's actually quite messy to do when you have an adult men's mountain bike, a lady's road bike, and a kid's road bike. I don't suppose you're anywhere near Cumbria? You could try ours. If you know anyone with a similar style of rack, it's really well worth playing with it! I saw a really clever one the other day, made by Westfalia. Probably pretty expensive but it folded up vertically against the back of the car when there were no bikes on it. Really handy because otherwise, when you park up somewhere and go for a bike ride, the thing sticks out a mile and you can't get into many parking spaces.