The reason is because it saves the team money. Rather than paying the full amount to get to the floor, they only need to pay the prorated amount to one player.
In this case, the Sixers were $11 million short of the floor before the trade deadline. They traded Ilyasova's ~$8 million expiring contract for Splitter's ~$8 million expiring contract + a 2nd and a 2nd swap. Technically those contracts would offset, but because Splitter's contract is covered by insurance due to his injury this year, the Sixers won't have to pay the $3 million prorated that they owe him for the remainder of the season.
Similarly, by acquiring Bogut to get to the floor, they don't have to pay the amount the Mavs already paid to him this season. When they buy him out they'll only need to pay the prorated amount, so ~$4.5 million.
This means that before the trade deadline the Sixers would have needed to pay the 15 players on the roster $11 million at the end of the year. Now they're at the floor and saved $3 million on Splitter + $6.5 million on Bogut.
So unless they plan on using all this money they saved to good use in the future, it's just a pointless money making scheme.