Chords

What you mention here are effectively the degrees of the scale. Out of these degrees you can build chord out of the notes in the scale. In C major I c major etc. What makes this useful is that it is a way write down Harmony and you know the chords with a minor or major tonailty. It also will go you finding the key signatures since a dominant chord always is the V chord so seeing a Dominant chord in a chord progression will mean that would be the V in the chord progression and from that you can pretty easily get the key signatures of the chord progression.
Also it helps you with finding cadences. There are 4 cadences.
Perfect cadence(V-I)
Imperfect cadencr(any chord followed by V (usually I-V, ii-V, IV-V))
Plagal cadence(IV-V)
Interrupted cadence (V followed by any chord(usually V-VI))
Each if these cadences had it’s specific role in music. Perfect and Plagal cadence give a sense of completion. Imperfect cadences give the impression that the music is going to continue. Interrupted cadence leads the listener to expect a perfect cadence however instead of the tonic there is a different chord.
Hope this helps! If anything is still unclear or you still have questions feel free to ask!