So, Smith is a smart guy, and what he has tried to do, and has done, is to anticipate Trump's moves that he would use to delay the case. 

He knew that if the case were filed in DC, Trump's attorneys would file for a change of venue to Florida and that would take time and delay things. Because the obstruction crime was committed in Florida, Trump's lawyers likely would win that challenge. So, rather than wasting all the time for Trump's lawyers to move for a change of venue and have that approved, Smith decided instead to just go right to Florida and file the case in the jurisdiction Trump would have wanted it moved to. 

So, it was just a way to avoid that delay. Smith also tried to get this immunity case in front of the Supreme Court last December, again trying to prevent Trump using it as a delay. The Supreme Court denied Smith's request and allowed Trump's attorneys to wait to first move it to an appeals court, where they expected to lose, then appeal to take it to the Supreme Court, where they sat on it until the last minute so the case could never come to trial before the election and then ruled Trump does have vast immunity for anything he did while in office...so, worst of all worlds and a blatant political move on the part of Roberts and his court.