The gradients serve two purposes:
1) to indicate the 'direction': dark to light or no ink to 100% coverage ink. (This is not implemented in PhotoLine or most other image editors, except Photoshop and Corel Photo, I believe)
2) it's simpler to visually tell where the right top range ends, because the triangle indicates this in the gradient.
Actually, when I worked in Photoshop I always preferred the triangles to set the range. And I find this one of those daily inconveniences in PhotoLine, because I keep missing the main curve handles due to them being too small - until now.
I quite like the triangles. I hope they stay. It's an improvement.