* is a wildcard. It just allows Salesforce to map the URL to your class when a parameter is included. To use your example,
can map the URL
where your class can parse and utilize the parameter value - an Account Id or External Id, for example.
Interstitial wildcards do the same thing.
would map the URL
to, for example, return data about the Contacts associated with the Account identified by the value
For more, see
URL path mappings are as follows:
The path must begin with a '/'
If an '*' appears, it must be preceded by '/' and followed by '/', unless the '*' is the last character, in which case it need not be followed by '/'
Salesforce does not parse the URL into parameters for your class; you're responsible for inspecting the incoming request and parsing and validating any parameters you expect. If your web service does not accept any parameters, I don't see any reason why you'd use a wildcard in your URL mapping.