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 as well as the iii.
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 through 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 can go directly to viidim and V or move towards the viidim and 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.