When to use RDS service in amazon? I have mysql installed in ec2 instance

I have a dynamic website and I need to migrate to AWS. I am new to AWS and linux. I have a doubt while setting the environment. I have installed Mysql,Phpmyadmin separately.

I have the following questions :

1. How to connect this installed Mysql with this installed Phpmyadmin ? How to access Phpmyadmin through browser in aws.
2. Why do we need RDS then? Do I really need RDS instance?

Please help me..