Defining the variable value before use

While it is not enforced in some languages, a variable must have an initial value in Rust, even if it is zero. This is good practice, and also helps when it comes to debugging since all variables have known contents. If they didn't, there'd be a risk of undefined behavior.

Undefined behavior means that what the program does is anyone's guess. For instance, if variables did not have initial values, their values would be whatever happens to be in memory at the time that the value is allocated.