The first and likely most obvious reason to utilize a two handed cast would be in an environment with limited to no room to allow for back cast. On streams with high banks or those dreaded trees and bushes that singlebarbed likes to get snagged in, utilizing a cast without having to perform a back cast is good news to your fly box. While a simple roll cast would serve the purpose here, sometimes you are presented with a situation that does not allow for a traditional roll cast to be carried out. If you've ever tried to make a roll cast alongside a bank with vegetation adjacent to your casting hand, you know how frustrating it gets when your fly line tangles in some grass, weeds, or branches. It downright sucks. This is a case where a backhanded roll cast would work great, or a single spey on your off hand.
Another obvious benefit of two handed casting is distance. Utilizing spey/skagit casting techniques can easily boost casting distances, enabling you to fish waters unreachable to other anglers fishing traditionally fishing their single handed rods. Unless they are Tim Rajeff, you should be able to easily outcast them. The one caveat to distance casting is that you may find yourself overlooking the near water, in attempts to reach the far bank. Coupled with having to mend line over multiple, dynamic currents, this can pose a dilemma to the two handed caster. A longer rod may enable you to high stick nymph runs longer, and may reduce the need mend, but there is a note of caution that fighting fish with a lot of line out may disconnect you from the fish. Especially when the fish is also fighting the resistance of your fly line against the current. I'd like to add that one underutilized application of the long cast, in my opinion, is lake or reservoir fishing. You should be able to cover a lot of water on the lake using two handed casts.
One benefit of the two handed cast is that it is easier on you shoulders and upper body than traditional single handed casting. This may be appreciated by you
An oft overlooked advantage of the spey/skagit cast is it's resistance to wind. Compared to other style of casts, two handed casts seem to cast better for me under control in windy conditions. Not having to aerialize a forward and back cast minimizes the effect wind can play out on the prairie here in Northwestern Minnesota. Wind is always an issue here. If I had to cancel an outing just because of the wind, I'd never get a chance to fish out here. It's just part of the environment. Utilizing skagit style casts in these situations have really benefited me. I am able to place casts mid-stream that would have otherwise blown back in my face, using a snap-T or a double spey cast. The line speeds of a two handed cast are incredible and play a big part against fighting the wind, much like a double haul, but without out all the effort.
So, I've pointed out several advantages of using two handed casting techniques, and a couple of switch rods to utilize those casts with, now what? How do we perform those casts? I'm no spey/skagit expert, nor will I pretend to be. If you want a source of two handed casting instruction, check out Sexy Loops or Spey Pages. They have loads of good stuff to fill your brain with. I will, however, explain the main differences between the spey and skagit styles of casting and why you may want to utilize one over the other. Until then, cheers!
More to Come: The fundamental differences between spey and skagit style casting and their applications.