The const keyword allows you to specify whether or not a variable is modifiable. You can use const to prevent modifications to variables and const pointers and const references prevent changing the data pointed to (or referenced).

Should I always use const?

i assume this question will be closed, so quick answer, you shouldn't always use any of them, you should use those types as to the need of the variable, const mean that this variable will not be changeable in the app, so if you see this variable you know it will not be changed, and its a constant, let means this will ...

Why do we need const?

We use the const qualifier to declare a variable as constant. That means that we cannot change the value once the variable has been initialized. ... For example, if you have a constant value of the value of PI, you wouldn't like any part of the program to modify that value. So you should declare that as a const.