You don't need either of them if you are attempting to identify new systems. You identify the new system first, and then enter information about it in DSPlus.Sites or DSDPLus.SiteLoader.
Read the .TXT files to learn more about SiteLoader. Populating DSDPlus.Networks and DSDPlus.Sites (for P25 systems) will make the system name and site names show up in DSDPlus. If you use DSDPlus.SiteLoader, it will allow you to quickly select a specific system/site to monitor. In either case, neither is autopopulated and you must know about a system and add its information before those two files are useful in any way.