The MLB draft is a bit different from the drafts held in the other major sports in that the teams don't often pick a player at a position of need. Rather, because of the length of time that many players need in the minor leagues, teams will pick the best player available.
Players can change positions, and it has happened often in the past. Bryce Harper was a catcher in college, and now he is an outfielder. College starters can become relievers, and pitchers who were relievers can become starters.
In identifying the biggest need that each team has in the first round, it is important to try to look three or four years down the road. With that being the case, many of these teams will not pick a player in this position of need and instead will go with that top guy that is on the board when their pick comes up.