There's one monopoly that matters in Monopoly: the houses themselves.
The game only has 32 houses. If you get two 3-property monopolies and build four houses on each one, forgoing hotels, you have 24 houses and everyone else is fighting over the remaining 8. If you get max out houses on two 3-property monopolies and a 2-property one, the game is yours regardless of what anyone else has.
That’s also part of the logic of locking down Baltic and Mediterranean. It’s cheaper to get the set, it’s cheaper to get the houses. You’re not going to make a ton of money off them but now at least 25% of the houses in the game are friendly.
I usually did better playing this strategy. Even though winning with houses on park Place is more fun, houses on Mediterranean ave are more certain.
They also make it harder for your opponents to keep their $200 when they pass Go, especially when you throw in the income tax space. Anyone who's on the ropes has a harder time coming off of them this way.
That, and if they're all built and someone has to sell one, it gets auctioned off. If you're building on green where houses are $200 anyway, you're willing to pay more for them than someone building on light blue where they're normally $50.
The game only has 32 houses. If you get two 3-property monopolies and build four houses on each one, forgoing hotels, you have 24 houses and everyone else is fighting over the remaining 8. If you get max out houses on two 3-property monopolies and a 2-property one, the game is yours regardless of what anyone else has.