Normalization: How To Deal With A Field That Does Not Have Any Associated Data

Hello.

I have a simple modeling question. Lets say I have the following table structure:

Table 1
-------
Cust_id
Cust_name
Phone
City
Supplier_id
Dept

Table 2
-------
Supplier_id
Supplier_name
phone
City

Table 3
-------
Dept


Table 1 is M-to-1 with Table 3. Table 2 is 1-to-M with Table 1.

I have seen such as schema and do not understand why there needs to be a separate Table 3. Can someone explain why Table 1 alone would not suffice and Table 3 would not be required?

Basically, can someone explain the rationale for normalizing tables when there do not exist sufficient associated data for the field for which a new table is created (i.e. in this example, if Dept had a lot of other fileds associated with it such as Dept_desc, Dept_Mgr etc , then I can see the value in creating a different Dept table and joining it with Cust table through a Dept_id field).

A similar such design exists in the standard "Address Book" sample schema that ships with Access (the analog field in question there is Role).

Thank you for your response.