vsiva23,
1. What is the purpose of this form?
2. If you change your emplyeer, do you need to file this I-9 Form again?
Yes. When you join a new company, the HR department will first verify if you are eligle to work. They ask you to fill out the I-9 form to check your work eligibility.
3. Where should this document is saying, with EAD receipt notice you can continue your employment without EAD card?
As a part of proving your eligibility, you need to pick any column (between A, B and C) and provide at least 3 suuporting documents. One can be your SSN card, passport/driver's license and your EAD card. The form does not specify EAD as such. But "work authorization" technically means H1, Green Card, EAD, etc.
