It's a good idea to always give all your tables an auto_incrementing primary key.
You need it when SQL has no other ways of identifying which row you want to edit.
For example, if you have a table like this:
name | price
Apples | 54
Pears | 62
Potatos | 54
Apples | 54
and you want to change the second apple entry to bananas, the SQL would be:
UPDATE table SET name='Bananas' WHERE ... where what?
There is nothing that uniquely identifies the second Apples record, SQL cannot do this, it must fail.
Now if you have an auto_increment PK, the table looks like this:
ID | name | price
1 | Apples | 54
2 | Pears | 62
3 | Potatos | 54
4 | Apples | 54
Now you can identify the second Apples record by it's PK value:
UPDATE table SET name='Bananas' WHERE ID=4;
get the picture? :-)