Variables and functions help

I don’t understand why you would need a [let] function in front of a variable.
Here’s a link to the lesson:
https://www.codecademy.com/courses/introduction-to-javascript/lessons/variables/exercises/const

I understand why [const] would be important, but I just don’t understand [let].
If you create a variable, lets say dog = beagle, and you do a [console.log], it will say “beagle”. Then, after that, you want to change what the variable equals, you can just put another dog = boxer after the first one.
Then if you do a [console.log] it would say boxer, but the first one would still say beagle. So what’s the purpose of the [let] function?

Consider this example:

const dog = 'beagle';
let conduct = 'wild';
conduct = tameDog(dog);
console.log(conduct) // tame;

To know the current state you have to have a variable that can be changed. If you had const tame and const wild declared separately, you wouldn’t know if the dog is dangerous or not.