It changes the order of joining.
This:
select * from Table1 a
full outer join Table2 b on a.id=b.id
full outer join Table3 c on b.id=c.id
full outer join Table4 d on c.id=d.id
is equivalent to
select *
FROM
(
(
Table1 a full outer join Table2 b on a.id=b.id
)
full outer join Table3 c on b.id=c.id
)
full outer join Table4 d on c.id=d.id
While
select * from Table1 a
full outer join Table2 b on a.id=b.id
full outer join Table3 c
full outer join Table4 d on c.id=d.id
on b.id=c.id
equals
select *
FROM
(
(
Table1 a full outer join Table2 b on a.id=b.id
)
full outer join
(
Table3 c full outer join Table4 d on c.id=d.id
) on b.id=c.id
)
This doesn't matter when you are doing inner joins, and in that case the optimizer may also reorder the joins for efficiency.
This is not the case for outer joins.