There was a lot of work before Lilienthal but he was the first to demonstrate heavier than air flight, which was a really important step in the history of human flight. The Wright brothers basically copied Chanute-Herring's design, as another user already pointed out. Their design was in turn an improvement on Lilienthal's. The Wrights had studied all of that, they said that Lilienthal inspired them to get into flight. So maybe without him, they would never have worked on it.

That said, there were many others working on gliders both before and after Lilenthal. It's interesting seeing the progress over time, in retrospect. Makes you think about our modern age. The general public was extremely skeptical that "flying machines" would ever work. It's still the same today, with out modern tech innovation. We could have so much more progress if the majority were more open minded and into experimentation. They mock inventors until the invention actually works, and then after a few years everyone pretends like it had always been obvious.