What it basically boils down to is this. Assuming equal skill and not having one team drastically outpick the other, you will never take a point from someone who owns it. The turret, even late game, with garrison does enough damage to prevent this, especially with tanky derps and AoE CC.
So if in that initial windmill fight, one team has revive and the other does not. For however many minutes, they have extra health and there's almost no way to initially beat them. They own it and they won the game. You finally beat them 9 minutes later? They all come back while 2 of you are still dead and all have 400 extra HP. GG you held top for 30 seconds.
And then on bottom, you beat the guy bottom and push his turret. JK, he spawned and is now moving faster than a bat out of hell. He kills you and takes your turret. It's basically a spell that on its own wouldn't be that good, but when a whole team takes it, then you have to take it to counter.