While I'm clearly out of my depth with this question, let me give you a few pointers.
(TL;DR version: Thogal is cultivation of "suchness", aka tathata).
First, you need to understand what Dzogchen, or Great Perfection, really refers to. Basically, this is the direct realization of the Heart Sutra experience. In other words, you need to clearly see that what you imagined to be the solution, is really a part of the problem. To quote Dogen,
The water is clean, right down to the ground,
Fishes are swimming like fishes.
The sky is wide, clear through to the heavens,
And birds are flying like birds.
Second, you need to have experienced a glimpse of your "true self" or "true nature". This is separate from the above, and requires a teacher to push you off "the cliff of your ego", one way or another.
Then, once you've seen your true nature both ways, you will have no doubts as to why you want to cultivate it, neither at conceptual, nor at experiential level.
In my limited understanding,
Trekcho ("cutting through") and
Thogal ("transcending the peak") are two aspects of cultivation of the true nature.
Trekcho is fearlessly cutting through all your B.S., while
Thogal is learning to spread the wings of natural spontaneity, with no sense of observer, no boundaries -- the space is dancing by itself.
To give a totally unrelated quote (at risk of getting killed by lightning for messing with the will of gods LOL!)
Dance like there's nobody watching,
Love like you'll never be hurt,
Sing like there's nobody listening,
And live like it's heaven on earth.
Or to quote Chogyam Trungpa,
You need to develop a state of mind that is cutting through fundamental spiritual materialism. Beyond this, there is nothing to cut through.
At the same time, we see that destroying spiritual materialism is itself a form of spiritual materialism -- and destroying that destroying is spiritual materialism as well. Finally we begin to see that even the idea of cutting through becomes questionable.
The mandala practice of Dzogchen is to just experience things as they are, fully and completely. You realize that there is nothing to hold on to, and at the same time, there is no way to make the groundlessness another ground.
You experience the vastness of space and the sense of no boundary. You experience the playful aspect of space, in which there is no hope or fear, pleasure or pain, which is the best dance. And you experience the pragmatic aspect of space: the fact that space is hopeless, and it does not mean anything very much.
This discipline of noncaring is not so much the accomplishment of the path, but rather the style of the path.
As you can see, in all truth, I have no idea what I'm talking about. But here are a couple of links that may shed more light on this obscure topic: