What is the advantage of using World wide mode over setting domain manually?

Answer

When radio is set to worldwide mode, it first tries to adopt 802.11d country code if qualified (adoption of country code is not just affected by AP it attempts to connect but by all the AP it hears along with various factors). If radio adopts a 802.11d country code, it will behave like you already set a domain in radio. If radio does not adopt one, it will stick with "pure world wide mode" which in general produced less Tx power and limited channel set to be connected. If you know which country the devices will be deployed, it is simpler to set the domain in radio manually as you do not need to consider environment of the site. If devices can be anywhere in the world, using worldwide mode is fine too as it can still connect/roam (possibly with less power though).