Serves to Ionize the Air?

Is this an accurate statement? My understanding was that once the stepped leader connects with a positive streamer or another object the channel is formed and the air is ionized. Why then does the air need to be re-ionized by the Dart Leader, if it's already ionized by the initial stepped leader and the lightning flash? I think the article needs toe be fleshed out a bit more and citations added. I'm sure plenty can be found at meteorology sites... MAybe NOAA if thay have a glossary, or AMS, etc.