Using count on a join, group by required?

Greetings!

First: This is working, I just need a clarification on concept, so, it
is not necessary for you to look deeply at the SQL statement.

I have this:

Table: products that references manufacturer via
products.manufacturer_id to manufacturer.id (not important, just
informative).
Table: product_serials that references products via
product_serials.product_id to products.id

And I wanted to get this output:

Product ID, Product Name, Product Code, Manufacturer ID, Manufacturer
Name, Number of Associated Serials.

So, I build a query for that:

select products.id as product_id,products.name as name,products.code
as code,manufacturer.id as manufacturer_id,manufacturer.name as
manufacturer_name,count(product_serials.product_id) as num_serials
from products left join manufacturer on
products.manufacturer_id=manufacturer.id left join product_serials on
product_serials.product_id=products.id group by
products.id,products.name,products.code,manufacturer.id,manufacturer.name;

And it works, it gives me something like:

product_id | name | code | manufacturer_id |
manufacturer_name | num_serials
------------+----------------------+---------------+-----------------+----------------------------+-------------
17 | THE product | 1235711131719 | 19 |
THE product's manufacturer | 5
6 | Car Battery 500A 12V | 7591512021575 | 8 |
Acumuladores Duncan, C.A. | 11
1 | Test product 1 | 123456789012 | 1 |
Test Manufacturer | 6

Which is correct, and exactly what I wanted.

So far, so good. The thing is: the group by clause, I had to add it
because the parser forced me to, because it complained like this:

ERROR: column "manufacturer.name" must appear in the GROUP BY clause
or be used in an aggregate function

and I had to include *all* the requested columns on the group by
clause, can anybody tell me why? or at least point to some doc that
help me understanding this?

Thanks in advance,

Ildefonso Camargo