Well, there is a huge difference between having 9 players on your team and a weak one vs 9 players and a strong one.
The strong player will sure make a difference, but 5 strong players will probably hold them down enough to not make a huge difference.
Now, the weak player will make mistake after mistake, and the average enemies will pounce on that. They will get a good lead, and that lead will get them far more objectives and control.
Knowing that you can effectively make the game 5v4 because of the weak player automatically puts you in a huge advantage. A strong player will still count as one though. The weak player will probably be useless, sometimes even it would be better if they were AFK.
I can remember a lot of games where I was doing good but I would have a weak (sometimes inting) jungler. Those games, I really wish my jungler would be AFK. He went 1/19 (resetting his gold given at 9 kills, granting even more money), and made the 4v5 even harder, negating any lead I had gained. If that guy literally AFKed, I would probably have chances to win.
The 5/0 Jax top is strong, but it doesn't mean that his enemy who is 0/4 or something is weak. Maybe he was double ganked through fog of war, maybe he got outplayed or something. Shit happens. An average player being 0/4 will still be useful later. They will still try to find ways to come back, and do something meaningful, even if the enemy has a fed Jax. Sure it's hard but it happens. Happened to me a lot of times.
Now, if that is an average Jax who is again 5/0, but that enemy laner is 0/4 but obviously a bad player (takes bad trades, can't cs, doesn't ward, bad runes / items), then you know the game is over. Jax's team will play around him, they will get fed on that 0/4 guy even more, take his tower, take another one and so on, and there will be literally no counterplay. That's the difference.