Well just for an example the standard side control escape in Bjj requires you to turn into your partner (either via bridging or shrimping) in order to bring your inside knee into their hips to create space. However, when I sparred someone (a former wrestler who was incredibly good at applying top pressure) and I fell into the bottom of his side control my partner drove my upper body forward so that I couldn't turn into him and create the space, even with the arm frame. His driving pressure was so strong that my frame actually worked against me, and the blade of MY hand left a mark on my face.Thus, I immediately realized that that particular side control escape wasn't applicable for every situation and I needed a solution to the problem.
That solution came with a different escape that allowed me to get out of the position almost perfectly. However, I wouldn't have figured that out in a no sparring/light sparring situation. It required hard sparring to figure out, because it was hard sparring that made me realize how dangerous that position could be if I'm dealing with obscenely strong top pressure. I'm glad I found that realization against a training partner on a mat than a mean dude on concrete.
Now, I will concede that I was a bit harsh in my earlier remark. You can definitely gain martial skill, but you develop better martial skill more quickly through hard sparring.