Special case of Equi Join

Question

I came across this particular script which uses a special form of equi join.

SELECT * 
FROM 
per_assignments a, per_assigment_types b
WHERE
a.assignment_status_type_id + 0  = b.assignment_status_type_id

Why is the zero added in the equi join? I came to know that it has something to do with avoiding index search, but still can some one explain the complete picture of the same. Thanks in advance

Edit :

It's not something which is related to the Table / Column declarations. As far as I know it's something to do with SQL tuning.

This is what I found :-

1. This is used in smaller tables.
2. Instead of doing an index search as done normally, this would search the complete table in one go.

But I really don't know exactly what's the difference with a normal equi-join, moreover how indexing affects performance.

It would be really helpful if some one could describe within the particular context and also let me know if my findings are wrong. Appreciate your time and effort for the same :-)

Column Description:

The assignment status type Id's in both tables are declared as NUMBER(9)