Although EDA toos is convenent for us to design chip.
But it is not very smart.
We must make some commands or constraints.
EDA tools can not know how much driving strength will be applied in PAD.
It is dependent on PCB and output driving device.
So insering IOPAD by hand is necessary.
To be more compatible with the environment of your PCB, the IO pads provided using synthesis may not be the best method, so some times some very timing-constraint IO pads shall be replaced or inserted by hand. And sometimes these IO pads are customerized according to your PCB environments.