In modules, we typically set up applications: most of the time this is done by installing packages, configuring files and managing services.
We can write a module that makes exactly what we need for our working scenario or we can try to design it having in mind that people with different needs and infrastructures may use it.
These people might be us in the future, when we'll have to manage a Puppet setup for another project or cope with different kinds of servers, or manage unexpected or nonstandard requirements, and we might regret not having written our code in an abstract enough and reusable way.
Basically, parameters, being our API to the module's functionality, are needed for exactly this: to let modules behave in different ways and do different things according to the values provided by users.
Hypothetically, we could enforce inside our code exactly what we need and therefore do this without the need of having any parameter. Maybe our code could be simpler to use...