By proliferating the counters on Agents ToolkitAgents Toolkit, they can make extra counters and move those extras off to make the Toolkit act more like a Bag of Holding from Dungeons and Dragons
in cEDH, Blood Pet is often found in Krrik, Son of Yawgmoth builds as a way to net an extra mana for 2 life, but also sees a lot of play in Inalla, Archmage Ritualist and Ad Nauseam lines to rush out these expensive spells or aid in comboing off with other cards to win the game
The addition of vigilance makes it that much stronger, but can block an additional creature each combat is such a unique effect, and in the format where most people claim to slot cards for redundancy, or still wrongfully cling to the 8x8 theory , youd expect High Ground to show up more
Note we're looking for the word destroy today, so no exile/sacrifice effects like Realm Razer or Cataclysm
Regeneration cannot save the creature with toughness 0 or less
The last is the nasty one that puts this deck over the top as a life-gain trigger for combos that you can never stop
