Sabine Hossenfelder at NPR:
Newton's law of gravity — remember that? The force between two massive bodies decreases with the inverse square of the distance and so on?
To use it, you need a constant, "Newton's constant," also called the "gravitational constant," usually denoted G. You can determine G to reasonable accuracy with a few simple measurements.
Once you have fixed the gravitational constant, you can apply Newton's law to all kinds of different situations: falling apples, orbiting planets, launching rockets, etc. All with only one constant!
This ability to explain many superficially different processes is what makes natural laws so powerful. Newton's contemporaries were suitably impressed.
After Newton came up with his equation, he could have reasoned: "Since I don't know this constant's value but have to measure it, the constant could have any value. So, there must be a universe for each different value. I conclude that we live in one of infinitely many universes – one for each value of the gravitational constant. I will call this collection of universes the "multiverse.""
But he didn't. Newton was famously minimalistic with his assumptions and even refused to speculate whether there were deeper reasons for his law of gravity, arguing this was unnecessary. "Hypotheses non fingo," he wrote, "I feign no hypotheses."
But that was then.