Hi, I can help answer your question.
You don't need a primary key for your lookup field as you describe it.
The primary key comes in handy when you want to change the words in the Lookup field and are linking by primary key. This way all the entries get changed instead of having to change all rows in the table.
Example:Primary key: NoneLookup: My Client Name
If you wanted to change the lookup later on to "My Client Full Name" you can, however, you will then need to change the text in all your client table since it's not linked by primary key. If you had a primary key linked to the lookup, you would just change "My Client Full Name" in one place and it will automatically show the correct data in the client table.
The primary key is usually an auto incrementing number that you don;'t have to worry about.
Thank you, XXXXX XXXXX the "when" to use a primary key.