Ids is correct! Roman Numerals can tell alot about a song without actually stating a home key for the piece. Kinda allows for a group of musicians to key change quicker instead of reading chord charts with ‘Capo 1st fret’ written on it.
We can establish a few things from Roman Numerals. One of the key points to note is that we can identify majors from minor from diminished from augmented and so on. If we had a major scale in roman numeral format, we would have I ii iii IV V vi viib5, the lower case would represent minor and upper case would represent major. Every scale has roman numeral format.
From this you can work out what progression is being used pretty effectively, and sometimes even work out the movement of the chords and if chords have been borrowed from another scale.
You can also work out what chords could be substituted for another chord with a similar quality it may share. For example, replacing a ! with a iii or a vi; or replace a V with a viib5.
Quite alot going really, but can be quite effective and quick once you understand the idea.