Yeah, haven't they always done it to prevent/minimize the chance of "tactical" play?
In terms of fairness, there's more too it than just situations where teams could collude to get a result beneficial to both (I assume that's what you meant). Even if only one team has a chance of progressing from the group, by playing their match later they could be considered to have an advantage in knowing the exact result required of them.
However, I think there are a couple of more crucial reasons for playing simultaneously:
- It means all teams know whether they have qualified at the end of their match, so players and fans are able to celebrate straight away.
- It reduces the chance of a game starting with both teams already out of the tournament. (Only possible if both teams have lost their two previous games.)