Well, when casting on a more vertical plane, its generally easier to be accurate because the turnover is more in line horizontally with the direction of the cast.
With a more sidearm angle, the casts turns over in a sweep that's more parallel with the water's surface, and where the fly comes to rest is a bit harder to control.
When casting sidearm, its possible to give the fly a vertical turnover, but you have to add in what's commonly known as a curve cast. When performed on the vertical plane, I've also heard this one titled a "tough right hook". When tilted 90 degrees, it allows you to send a cast under overhanging brush or limbs, and then kick over vertically at the last minute to help it land right where you wanted.
Shortly before you stop on your forward stroke, roll your thumb slightly to the inside. In other words, after you've had your palm facing up while casting, finish the stroke by rolling your thumb up. Its tricky, and takes practice for sure, but its the best weapon I've got for getting a fly to where the big ones live.
Ed Jaworoski explains it better than I (though he's talking about casting in the vertical plane- Casting Curves | Fade your cast to the right or draw it to the left with these simple steps| 1