Stacking creates more creeps at no cost, which basically introduces more potential gold and XP into the game (hopefully gold going into your carry). At higher levels of play where the smallest of advantages matter, it can be huge (a lane can be considered lost after 1 kill in pro games because they can use the XP advantage so well)
If you take a hero that gets an early Helm of the Dominator, it is standard to dominate the big Satyr (the one with HP regen) and use him to stack your Ancients over and over again (you need a Satyr specifically because he regens the damage he takes pulling the ancients). Whenever your carry is strong enough to clear the stacked camp, it can be 1000+ gold.
EG ran a play like this against Na'Vi where they stacked a ton of ancients, then had Tinker drop double March of the Machines, Dark Seer dropped Ion Shell on him, and they smoked. Step Three = Profit as everything died helplessly.
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