The I scale degree is where music starts and ends. It is called the tonic.
The vi scale degree can also serve as the tonic.
Movement from the I and vi scale degrees is always to the V scale degree. The V is called the dominate. It creates harmonic instability and leads back to the tonic. When I moves to vi and back to I it is normally to a I6.
But like the tonic, the V scale degree also has another chord that can function as a dominate. It is the diminished vii. So I and vi will always move towards the viidim or the V through the pre-dominant .
The IV scale degree is called pre-dominate meaning it leads to the dominate. The ii scale degree is also pre-dominate. The ii in first inversion (ii6) is actually more common than the IV.