Ballasted deck bridges -- the part of the bridge that holds the ballast (on a modern metal bridge) is called the "deck pan" -- offer superior running characteristics to non-ballasted bridges.

Consider this, if you need to make small adjustments in the track alignment on a bridge where the ties rest directly on metal or wooden girders, you have very few options. But, if you need to make adjustments on a ballasted deck bridge, you simply dump a little more ballast and run a tamper back and forth until you get the precise alignment you want.

Consider the fact that bridges or trestles may sink even minutely over time -- and you still have to have the track line up with the track on either side of the bridge.

I recently witnessed the replacement of a century-old railroad bridge, where a new ballasted deck bridge replaced an old girder bridge without ballast. See:

http://www.robl.w1.com/pix-6/clayt-br.htm

One thing that was done was to raise the level of the track just slightly. This was now fairly easy to do by dumping ballast on the approaches and the bridge -- and then tamping to get everything in line.

Sometimes a "straight" bridge or trestle will actually contain a curve, sometimes very slight. In that case, by tamping the ballast, you can get the needed superelevation on the outside rail -- something that's very difficult to do without ballast.

Hope this helps.

-- Ernest