It's a good way of easily differentiating between whether you're talking about Ireland the island, or Ireland the country, which currently excludes the six counties of Northern Ireland (NI) which are still part of the UK.
The term Ireland is often used to refer to the island as a whole, but obviously this can depend on the political leaning of who you're talking to.
Northern Ireland, depending on context and/or who is speaking can otherwise be referred to as Ulster (problematic, as the province of Ulster has 9 counties, only 6 of which are in Northern Ireland). It is also common in some circles to refer to the 6 counties currently occupied by the British simply as "the north".
In short, Ireland is a hot geopolitical mess and when talking about Ireland, it is useful to be as specific and as neutral as is possible.
So ROI/NI are quite useful shorthands.