the ModuleID is used to identify which data goes with with module 'instance'.  When you place a module on a page, it is assigned a unique 'ModuleID'.  Now, let's take the documents module as an example.  You place 1 instance of a documents module on page 1.  Let's say it is assigned a ModuleID of 1.  Now, put a documents module on page 2.  It would be assigned a ModuleID of 2.  Now, both of those modules store there data in the same database table, Documents, so without using the ModuleID, how would you know which documents should be shown on page 1 and which documents should be shown on page 2.

Now, if you only ever expect to have 1 instance of your module on a web site, then you wouldn't need to use ModuleID, OR if you wanted EVERY instance of your module to show exactly the same data, then you wouldn't need to use the ModuleID, but in most cases the use of ModuleID is absolutely critical.