There is an advantage to a flat dart throw which leaves the dart at 90 degrees to the dart board surface or close to it. Nearly all the leading players darts hit the board at a shallow angle close to 90 degrees or 90 degrees. A particular good example is Phil Taylor.
When the dart arrives at those angles there are less chance of a bounce out. A flatter throw is generally a harder throw. The end of the dart has a greater surface area to hit in the target and you are less likely to hit the edge of the scoring rings and bounce out. The face of the scoring ring is about 1 mm but the edge can be 2 mm.
The player with the steepest angle in the board of the professional players is Brendan Dolan. Sometimes they look about 45 degrees. I have always though he could do better to either throw the dart harder or flatter or change his dart set up so the dart landed closer to the 90 degrees. If you watch him he seems to get more than his share of bounce outs. The steeper angle indicates a softer lobbed throw so less potential penetration into the board and the end of the dart can more easily hit the edge of the metal scoring ring.