Roman Numeral Notation (RNN) is useful generally in understanding musical harmony, particularly for harmonising music, even more particularly for harmonising music liable to be transposed to another key - for example: where a singer's range can not cover a song in its original key, annotating the accompaniment with RNN simplifies transposing it to a new key.
Instead of or in addition to annotating with actual chords, a roman numeral shows which chord is intended: a I for the chord 'rooted' (based) on the keynote, or tonic, a ii for the chord rooted on the next note up the scale, the supertonic, and so on. It is common, but not universal, to use upper case roman numerals for major chords and lower for minor chords.
This table gives the RNN for all the Major keys - a similar one can be drawn up for the Minor keys, and both can also be used to transpose chord variants such as seventh chords:
|Common traditional folk keys in bold, f = flat, M = major, m = minor|
To transpose a chord, in the starting key's column, find which row the chord is in, move along that row to the destination key's column, and that is the transposed chord. For example:
If such tables simplify transposition, given their ease of use, why bother with RNN?
So RNN is worth learning, especially as an adult singer's vocal range remains more or less constant, and folk music tends to use just a handful of the twelve possible keys - it's fairly easy to get used to translating between RNN and these few real keys, possibly even well enough to translate on sight in real time.