Can you say what you mean by "significant?" Do you just mean why everyone mentions it when they talk about him? I suppose it's just because it is a pretty interesting thing to the modern history reader that he was a eunuch! It's typically made mention of for most eunuchs in history. There's a few eunuch historical figures where it's not mentioned, I've got a couple of composers listed in Grove Music Online (the standard encyclopedia for Western music) where Grove did not mention that they were eunuchs, which is more odd. Being a eunuch was a large part of Zheng He's life, and indeed most eunuch's lives, so we mention it.

It was not considered honorable to be a eunuch, but eunuchs did have a specific prescribed social role, which Zheng He filled through his life, even while he was making his voyages, which was to be a high-ranking servant. Eunuchs filled this role in many societies, not just China. Zheng He's close relationship to the emperor was due to his being a eunuch and therefore having constant intimate (household) access to him. Imperial eunuchs were also customarily well-educated in the Ming (and Qing) periods, high education and the opportunity to become close to the emperor often allowed individual eunuchs to amass a lot of power, for better or for worse. So, had he not been castrated, Zheng He's life would have been very different!

There's a decent English book on Chinese eunuchs in the Ming period, the logically named Eunuchs in the Ming Dynasty, Zheng He is in it!