If all you want is a drop-down of car names, you could just have it as a value list, with no table at all. Yes, if you want to have an ID number, it would be easier to have a table, but there is no need for it to be linked. Unless you have more behind this, like a conditional value list requirement...
You do not have to in every case. It depends on the design of your value list , the design of your layout and whether you record data in additional fields that also needs to be accesssible once a value from this list has been selected.
The relationship is not required in order for the value list to function unless you are setting up a conditional value list.
You might want to set up a relationship anyway. If you select an ID number from that table as is often the better design approach, then you'll need a relationship for many layout designs so that you can add a name field from your table of values to display the name of the item selected after the user exits the drop down field where they selected an item from the value list.