The lines feature is simply a setting to limit the number of smultaneous calls IVM can handle for that telephony device / voip account. Typically this deals with either VoIP lines or advanced dialogic voice boards (usually a standard voice modem cannot handle more than one line).
With some VoIP services you may be allowed multiple (eg 5) simultaneous communication channels for people to call in on. You can then limit the number of simultaneous calls IVM can take on that VoIP line (independent to the number of communication channels allowed by your service provider). You must however have some sort of hardware or VoIP service that supports multiple lines / simultaneous calls, you can't simply feed a single line through a voice modem and let it take multiple calls at the same time.
In some cases, you may want to use the feature to limit the number of lines with IVM even if your telephone service may allow for more simultaneous calls. For example if 10 VoIP calls were in progress at the same time, depending on the internet connection of the computer running IVM or Axon... there could be a severe drop in call quality, or break-ups in speech due to the strain of trying to handle and stream audio data out to the internet to so many sources. So as a safety feature you might place a limit to avoid problems like that.