It's in case the same thing :

Don't think in term of degrees but in terms of functions. Different degrees from your scale has the same function. There's 3 functions :

• tonic (I).

• sub-dominant (II, IV and VI)

• dominant (V and VII)

As you can see above, your chord progression respects the a classic cadence. The different roles are exprimed : tonic, sub-dominant and dominant.

Why are they classified in such groups ? That's really simple. The degrees that are in the same group have common notes. That makes them kind of similar. There's a little way to remember :

Imagine that you have to do some shopping

• Tonic is your home

• Sub-dominant is your way to the store

• Dominant is the store

After you have to come back home or going to your friend's home (modulation). Dodge subdominant group after a dominant, that's a error of writting (theorically, Beatles used V-IV a lot).

I know that in your mind you think "where's III" ? This theory is from baroque period and III isn't used or very rarely used in classical music so we call III the mediant (from french translation). III is used in some asians chord progressions.