Honestly I never use it.
Once you split your data base,  it really doesn't serve much purpose other than as visual aid.
Then you can print out for documentation.

You probably will get many other people that use it a lot but my advice - don't spend a lot of time on it.

Relationship design is the only place to set up "referential integrity".  Richard has a video on this:   https://www.youtube.com/watch?v=cuni2mmLPl8

Referential integrity is useful for preventing "orphan records."  Suppose you have a product table, and an order table containing products from the product table.  If the two tables are related together with referential integrity, Access will not allow you to delete any product in the product table that still exists in the order table, whether you are in table view, query view, anywhere anytime.  If you try, you will get a warning as shown in the picture below, anytime anywhere.  This is a simple (no code) and useful way to preserve the "integrity" of your data -- because the order table cannot have products that don't already exist in the product table.  This is a basic and important feature in any relational database tool.

What Kevin says is true in a single db on one computer.  
However, once you split a database in order to use it with multiple computers, that all gets thrown out the window. It doesn't apply across the split.  You have to handle the example Kevin gave manually.

It does apply if you split the DB.  If the tables in the back end DB have referential integrity, all the front ends that link to those tables will also need to enforce such referential integrity.  The relationship diagram in the BE will also carry over to the FE.

I think what Scott probably meant is multiple BE files.

Thanks all, this helps. I think I understand. I will refer to the Relationship panel as a visual reference only and relate tables through Primary and Foreign keys.