It's a guarantee that economics in terms of trade both ways will not be disrupted as neither part of the island voted in favour of Brexit. It is also a guarantee that at no stage in the future will terrorists (of both persuasions) be tempted to attack a dividing line between the two jurisdictions which is what happened in the past. Some of the worst atrocities in the troubles happened at or close to the border.
But if the UK doesn't put in a hard border, I don't understand what the problem is.