You could have a deck made out of cheap Vampires (both monetarily and mana-wise) and still perform great because of how easy it is to flood the battlefield
Our deck can naturally fight through creature decks thanks to Engulf the Shore and Crush of Tentacles, but neither of these cards helps us stay alive against a Turn 4 Aetherworks Marvel, so having access to main-deck counters helps shore up what would otherwise be a bad matchup
Its not great, but its likely a lot closer to Glimmerlight (a great common) than to the useless Bronze Sable
In a two-player game, Secret Rendezvous is worse than playing no card at all, but in a four-player game, its a great way to start an alliance to join up against a common enemy
You can have up to 15 different lesson cards in your sideboard in BO3, but that wont allow you to have any other regular sideboard cards
Having a smaller deck means having faster and easier access to combo pieces, powerful cards, and other win conditions