Kabayan,
The fact that your family was granted a residence visa that means you are financially eligible to support your family in qatar.
Salary cerificate and your bank statement is being presented to the immigration during processing of your family visa application and this proves you are qualified once the visa is released.
Affidavit of support is required for tourist or visit visa. Otherwise if you do, you will only be spending your hard earned money to pay in the embassy for nothing. And the staff in the embassy will never tell you because they will be benefitted from your ignorance.
What's out you will be feeding Lacoste in barong.