it is not really adding a pk as much as relational databases generally use them so you can easily connect to related pieces of information that are store in 2 different tables. so in general, table 1 would have a field called something like

car_id which is just a number, but a unique number in that that table.

in table 2 there is also a field called the same thing.

if you join the two together table 1 would always match to table 2.

Sometimes you have to use a 3rd table. Say you have a table that has your model years. You might have to match your table1 to model years table on car_id. Inside the model year table there is another primary key that is makes a unique number per model per year (e.g. car_year_Id) and that is how you get the price by linking table 3 (year models) to table 2(prices) using the  car_year_Id field.

I am not saying your DB is made this way, just that it is a usual process and if yours is set this way you just need to figure out which PK to link to which tables to get what you want.