I think the mention of preparation is for rhetorical contrast: the goats are surprised to learn that they haven’t been serving God; they would seem to be the sort of people who would be surprised to hear that they were going to the place prepared for the devil and his angels, too. The reversal of expectation is total.
Not incidentally, this parable–though strictly speaking it isn’t a ‘parable’ by genre of course–ends out a set of three parabolic warnings, where the other two warnings are obviously spoken to Jesus’ own disciples about being lazy and/or uncharitable and how such disciples will fare in the eschaton when Jesus returns. I think the exact same principle is in effect here, too: those who think they’re the first and deserve special treatment as such may be the last, and the humble ones who never imagine themselves being anything other than last may be in fact the specially honored first ones. The comparison of judgment to the devil and his angels would express this again: they used to be loyal servants of God but then rebelled, and so now have to go in for brisk eonian cleaning.
This is actually one of the places indicating some hope for the devil and his angels, by the way, depending on how far the analogical language of the punishment term ‘kolasis’ is pressed: it could very easily be employed as hopeful disciplinary chastisement, and sometimes was employed that way, not only in non-scriptural Greek but elsewhere in the NT, too–most pertinently at Rom 11, though the specific term isn’t used there. Branches that have been grafted out for punishment can be grafted back in again, and St. Paul explicitly warns those who are currently grafted in not to be disdaining the branches currently grafted out, not only because they may be grafted back in again, but because the ones currently grafted in may also be grafted back out again!! Not one of the places in scripture most conducive to a Calvinist soteriology, to say the least… But the overall thrust of Rom 11 is on the persistence of God to save those, in this case “all Israel”, whom He intends to save–as Calvs are very well aware and make sure to score points about whenever they can. (They just interpret “all Israel” to be exclusive of everyone who isn’t elected to be Israel, which does a lot of violence to St. Paul’s warning about grafting in and out.)
The universalistic hope of Rom 11’s kolasis imagery, taken in total, is thus applied back even to the devil and his angels via Matt 25.