On joining a virtual world it is usual for the developer to recommend a client ui (user interface), usually referred to as a viewer.
For Inworldz, the developer recommends the IW Viewer or Kokua
Creators /builders might want to use alternatives to access addons & for expediency. Some functions are not enabled in all viewers, and limitations imposed in some are not in all.
SL Viewer aka Linden Lab Viewer has 2 basic versions, V1 & V2. The latter is now at version 4, making it V4. Some old school users prefer v1, for which some alternative viewers retain the option.
Viewer choice might depend on a user’s main activities. For example Singularity is useful for some styles of building, Kokua and Phoenix for jewelry, Exodus for gamers & artists, etc. Also, some clients bypass certain restrictions imposed in world-specific viewers.