Functions have parameters so that they can be told to do something different when they are called from different places or different times.
In a program this small, there is no need to have any functions but main(), so there is no need for parameters. In more complex programs, functions are absolutely essential — programs would be huge if you couldn't reuse code to do more than one job and build up complex programs out of small, separable parts.
As an aside, there have been other threads with some suggestions for those who are new to programming (along side lots of conversation about how suitable Rust is or isn't as a first language -- which there is no consensus on). For example, and another, and surely others if you search.