Tim Neverett and Greg Brown explained the timing of it during their call of the game. Apparently, if a player is still in Major League camp after the halfway point (which is tomorrow), he is required to go on the Major League Disabled List if he gets hurt. If the player is sent down prior to the deadline, his injuries will take him to the Minor League Disabled List. That's important because if a player is on the Major League DL and he stays on it when the regular season starts, he begins accruing service time, which means arbitration comes sooner.
For example, if Starling Marte would have made the cut and broke his leg (which let's say knocks him out for the entire year) playing games with the Major League club this week, he would be placed on the Major League DL and would get a full year of service time with the Pirates without even putting the uniform on. That means his arbitration clock would start a year early and the Pirates would be in trouble down the road.
There are all kinds of these quirky roster management rules, most of which the casual fan has no idea about. If you wondering why the Pirates sent Marte down so soon, that's why.
I think everybody would like Marte to be able to stay up and face more Major League pitchers; he has been putting on a clinic all spring and it's been fun to watch. However, the risk of leaving him up is too high and it was time for him to head down. Hopefully Marte will keep his hot hitting up when the Indianapolis regular season rolls around so we can see him in the majors before too long. Marte could be a phenomenal player someday, and it's starting to look like that day is sooner rather than later.