Why does the article refer to third parties as ''gadflies'', and why is this something positive?

The article refers to third parties as gadflies because third parties often bring attention to political, social, or national issues that neither major party wants to discuss. After third parties bring public awareness to these issues, the two main parties begin debating them. An example of this occurred in 1992 when Ross Perot ran on a concern that the national budget deficit was a troubling problem. Although Perot lost, his candidacy forced the Democrats and Republicans to discuss and deal with the annual federal budget deficit.