Sometimes they are need on forms/reports so you can get their value.
Richard leaves them visible mainly for his beginner students for learning purposes.

I remove them in "finished production" databases.

Just to add to what Kevin said there is no harm in using them --  BUT the issue that a lot of new Access users bring forward quite often is they want perfect ordered numbers with no gaps for a customer for instance.  
Or they wish to start at a specific number.  A number of scenarios.  

That is where the Custom Numbering comes in and why we generally say "Not for you".  In theory it could serve two purposes if you understand the reasoning behind it - being uniqueness of a record.

Great, thanks for the quick answers. All clear. I was wondering if I was missing something.